When do I call priest for last rights?

This is up to your father. I am an atheist so I think I would not want you calling a Priest for me.
Your father's reaction to life may be more realistic that you can know. You are not yet 98. I hope to goodness I never have to go there. I am 78, able, well, walk daily and am very active, but I am ready, and I do not look forward to a slow inexorable slide down until every single thing is taken from me, including my mind. I am ready to go.
We are not the ones to ask. Your father IS. Ask him if he is sad, if things are hard for him, how he feels. Ask him if he would like to see a Priest (I cannot know how strong his Catholic faith is; I would think he would have long ago asked for a Priest if he wanted to. It isn't about last rites. As you know, those are no longer only for the dying). Ask him if he would like to see a doctor and see if there is some sort of medication that might help him to feel better. Ask him if there are any activities, anything from a ride in the country to sitting out back porch with a beer, that would make him happier. TALK to him. Not to us. Follow your Dad's lead. My father in his early 90s told me he was exhausted with life and ready to go. He hated that my Mom got him up and made him get on scale and eat and go on a walk. He wanted to go, to just be in bed. He told me all about his life, all he loved, the single "bad thing" he did (which was pathetically small). He told me he would do it for my Mom as long as he could. Get up. Try. But he was over it.
There are many choices now with your father. Ask him. Give him your frankness, your honesty. Look at old pictures. Have old memories and a giggle. Let him eat things that are bad for him (Want a bag of chips, Dad).
Wishing you good luck. So sorry for the pain. It is dreadful hard to hear the truth and see what it can be for those we love. You are kind and caring. Just do your best and that will be soooooo good.

Last Rites can be done any time.
Call your dad's priest and ask him if he wold like to pay a visit, preform Last Rites. If your dad would find comfort in it ask if the priest would come every week or every other week. He can pray, talk to your dad and if the priest thinks it is necessary he can preform Last Rites. Or the priest may preform the Anointing of the Sick until he feels that Last Rites are required. He can do both.
If your dad would find comfort in talking or visiting with the priest contact them and ask if they will do a Sick Visit. At this point your dad does not sound like he is at End of Life. He may feel better with some spiritual counseling if that is what he would like.

They no longer call them "last rites". Instead the priest will administer the "sacrament for the sick". This addresses both the prayers for life and healing and the preparation for death. You can also arrange for weekly communion at your father's home and a Stephen counselor (a layperson trained to assist with all kinds of emotional issues) through your father's church. These things should help to engage him and give him a way to share his concerns and feelings about this life and the next without burdening you with all of them. Your church may have other ways to support you and your father in this time, including caregiver groups and senior companionship groups. Call them now - not for his eventual death, but for support in his life and yours.

it sounds like he could be far from last rites if he is still able to help in any way around the house.

See what you can do to engage/interest him. Your description is also consistent with depression.

Napping is normal for elderly but naps of more than an hour can be detrimental.

Try to help limit his naps by offering interactive activities that appeal to him. Even in these days of Covid, there are still things you can do to entertain him.

Try with short activities, lasting only a few minutes and then slowly increase the time every day.

If he doesn’t read anymore, maybe you could read to him. What was his favorite book as a young man? Even a couple pages, with a short discussion may be engaging.

Could he focus enough to play checkers or a simple card game?

If his interest span is too short and he can’t concentrate on a game, try TV.

Instead of Forcing him to endure current shows. Short clips of programs from the internet that he connected with and enjoyed as a younger man. Johnny Carson? Lawrence Welk? Frazier? Married with Children? My Three Sons? See if you can pull out a laugh or a smile. Bob Ross (TV artist) may be enjoyable. He may pretend to be ignoring the TV at first.

If your dad liked to drive, take him on drives while playing music from his era. Satellite radio took Mom back and made her so happy. Maybe you could get him a treat along the way or at the end.

You could also take him to see (or Zoom with) a psychiatrist that specializes in senior care. A low dose antidepressant, or even a visit where he can talk about “his problems and fears” with someone fresh may help him greatly.

If he is then not quite ready for those last rites but feels inspired by religion, ask clergy or other congregational assistants to Zoom and pray with him. It’s not quite the same as an in-person visit, but that attention from someone different may help.

Forget about the etiquette of Last Rites. Your father has lived a good and long life. His body is slowing down. His numerous naps and resting after 10 minutes of sweeping is just fine. He would probably love a weekly visit from his priest--or others in the congregation. Give him love, give him the foods he loves, take him for short rides and be glad for every minute you have with him.

You can have last rites as often as you wish. In my dad's last months he had this many times and it brought him a great deal of comfort and peace. Your priest can also give a blessing of the sick. Twice my dad was in a catholic hospital and received communion and blessings everyday. It was wonderful.
God bless.

Imho, perhaps your father's place of worship's priest can be called to talk to him, to reassure him on any issues that dad is dealing with (or not). Btw, an elder sometimes predicts their own demise. My late mother did. Your dad has every right to tire easily, given his advanced age.

call today.

Grandma1954 has some good info. Letting him talk to a priest may uncover what his unhappiness is or resolve some issues. It can't hurt. There is also some good advice here about talking to his doctor about his depression - or his desire to longer be here. There may be a medication to help.

Thanks for all the various advice. Follow-up: I decide to ask him. Told him since he is waiting for God to take him, maybe we can get ready and call the priest and see if the priest could give you the last rites? He said " Hell No! "
We will be getting new caregiver - he loves cards and will have fun teaching her sevens.
Then I will visit in 3 months to see progress.