When is a good time to tell my parents they need to move into a nursing home?

Are they living with you and your family?
If they are living on their own you can't "force or make" them move.
Why do you think they need a Skilled Nursing facility? Would not an Assisted Living or Memory Care facility be better? Mom with dementia should be in Memory Care. In most places a spouse that has not been diagnosed can live with the partner with dementia either in Memory Care or in Assisted Living as long as the spouse with dementia will not wander off.
Have they seen any Assisted Living or Memory Care facilities? They are a far cry from what many people think of as "nursing homes". A friend of mine recently moved into one and I call it a "landlocked cruise ship" with the movie theater, activities, buffet meals I can tell you my Husband would have LOVED that!
If they are living with you it is more difficult. If they will not move out you may legally have to evict them.
Mom can not sign a POA, with the diagnosis of dementia she legally can not make that decision.
You could try to file for Guardianship of your mom. Then you could place her in Memory Care. I can tell you this would be difficult and would drive a wedge between you and your parents and any siblings you have. Not to mention it is not easy or inexpensive. (although cost comes from their assets not yours)
If they are in their own home you can do what you can to help them realize that they would be "safer" in AL. If you are helping them (propping them up) in any way STOP. Unfortunately sometimes it takes an incident that forces the change.

Big jump from living at home straight snf skipping as al.

Yes the law supports ur parents decision over urs. the law only responds to after something happened not what might happen.

Draw a flow chart of each step in aging process and what u can do to make every step work. And when current step fails they have to move to be next step

Living at home without accomadation - living at home with accomadations -al-snf

U going too my fast.. now u should be asking how can I make them safer/asier living at home..
When that fails go to next step

Same issue

I must wait for my mom to have a life-threatening health emergency to be forced into Assisted Living or nursing home

This will also allow me and my brother to untether ourselves from our mom while making sure her needs are being adequately met!

Its very difficult when parents need help. Maybe have someone talk to them.
You can File for guarandianship, so you can help place them
Or just wait until something happens, like a fall. Call paramedics.
Tell case manager they can't live alone.

Call social services for their county to discuss options and consequences of reporting them to APS. My stepFIL became a ward of the county because he had Parkinson's, was broke and wouldn't give anyone in the family legal authority to help him. He was expecting us to be orbiting around him, driving over at midnight on a work night to pick up his 6'4" body off the floor. Refused to fill out the Medicaid paperwork. Once there is a court-appointed guardian, no one else has insight or control over that person's financial or medical affairs. So, one day I discovered I was locked out of the joint account we had to pay his bills. There's much more "backstory" than what I've written. It was sad, but also a relief.

your choices are limited to waiting until an event disables one or both of them (fall in or outside of the home, dangerous use of household equipment, wandering and being unable to remember how/where to return home) OR acquiring legal documentation that one or both of them are unable to safely manage and provide for themselves.

If you see ANY careless or dangerous actions in their current home life, call their local emergency services. Do that as often as you feel they are in danger. Local legal codes vary, but they may be able to direct you to the next step in acquiring care.

You may contact one of their medical providers but unless you send a note requesting “screening” for geriatric cognitive loss, you will not be given information on your first try because of HIPPA.

If you are able to provide specific, objective information to their medical specialist(s), it MAY help you to further your case that THEY are unsafe and THEY need support that they are refusing.

I could have “disarmed” my tiny LO, but believe me she could do some damage when she swung that heavy four footed cane at me, until her anxiety FINALLY forced her to leave the little farm house with the almost totally inaccessible SINGLE BATHROOM, to enter a REALLY NICE Assisted Living facility, where her disabling anxiety/dementia was ultimately revealed.

Hothousflower and I share the same misery, and we understand FULLY, where you are. BE STRONG, BE LOVING, BE FAIR, realize that you are not able to do this without the anger and in my case, shrieking and yelling insults that come from being “outed” as too frail or sick to manage on their own.

Thinking of you and hoping that you can pull things together for them before worse happens.

Best thoughts to you and your husband!

My brother and I had to sit tight and wait, finally my mother had a slight stroke and was afraid to be alone at night, we sprang into action. I think it is even worse when there are 2 involved both thinking they can help each other, reality being neither can.

They are just being stubborn and if like my mother thought that the homes were like they were 50 years ago.

Here is the irony, my mother loves it where she is, new friends, activities, no cleaning, she says "I wish I would have done this years ago"...groan!

I’m in the exact same situation. I did tell my mother that their situation was not sustainable and they needed to consider alternatives, AL or nursing home. She was livid with me, still is. My parents refuse to leave their home. Accept there’s nothing you can do and just wait. I did insist that my parents have a medical alert system. There is a lock box with keys on the door for EMS. They did finally agree to 8 hours of help a day, not enough but something. So we wait. I just wish this situation was over already. I can’t stand it anymore. It’s been around 7 years now that my sisters and I have been dealing with this situation which is getting grimmer snd more depressing as each day passes.