How long can a person body maintain on a ventilator with their oxygen level at 75?

To the OP, I doubt you are appointed to make MILs decisions. She is seriously ill. Docs may recommend withdrawal of support based on the condition. At a week in ICU with low sats, it will be a while longer to come off the vent. With length of time in bed, depensing on age will cause muscle wastage. She might also end up in long term care. I hope your family gets some news from her care team.
For the time being, sats in the 70s do require sedation to fully rest the body and for her not to feel discomfort. I once took a Sat moniter in the Alps. When walking, my sats were 76 and it felt very uncomfortable

Please rely on the doctors and your knowledge of her health conditions to know if a reasonable recovery is possible. My son has many times had 02 saturations in the 50’s, lived long term in the 70’s with no intervention (his body actually adjusted and became used to it) However, he is far younger and there was hope of it getting better. There are consequences to such low sats that show up in brain function to widely varying degrees. The hospital has no incentive to keep this up, they will follow what the family wants after you are fully informed on what her recovery and future might look like. I wish you all peace

I so hope there is a Medical POA. This will tell you what her wishes are. If its says no vent, then it should never have been used. I think once it is placed very hard to get removed if the patient is out of it. If she goes on Hospice, it will be removed.

Talk to her lung Dr about this. My daughter told me under 90 was not good, 80 bad.

I disagree with SP that the ventilators are "big business" for hospitals. In fact, they cost horrifically. A hospital gets paid by DRGs (look up Hospital DRGs). If you come in with a diagnosis of Covid or or Pneumonia or of blood clot that gets a specific coding and a specific payment to the hospital. The code says how much money the hospital gets, PERIOD.
If the hospital gets the patient OUT the door before the number of coded days allotted they make money.
If the patient ends up on a ventilator the hospital LOSES big time. They are very expensive devices in terms of cost, ICU needs and etc.
I would, in fact, be VERY surprised if the hospital personnel is not already encouraging anyone who is Medical POA to get this patient OFF THE VENT... to "let them go". (I am not saying that isn't the right choice; it likely is).

Faith, you don't tell us a lot of information about the age, the general condition, or the diagnosis here for your loved one. For that reason only I couldn't hazard a guess here. You will be now in an ICU situation. The doctors there will advise you. They are likely ALREADY advising you. I would listen to them.
I would say that they are likely correct in their advice whether to wait a bit or to remove the vent.

As an old retired RN I will tell you these are dire and worrisome numbers. You best know the person currently on the vent, you know whether they are a "fighter" who wants to go all the way or someone who is more or less ready to go and would not want to be on heroic measures for long. At levels of 40 there may already have been some damage to the brain, either temporary or permanent in oxygen deprivation.

I am so very sorry. My heart goes out to you. I believe you are the best one to make this decision knowing all you already know, and with the advice of the medical team and I trust you to make this decision, tough though that is for your loved one. I hope you will update us.

Those ventilators are still big business for hospitals. Unless she was having trouble breathing I can't see why they would put her on a vent just because her levels were 40. They pulled that same crap with my MIL when she went to the hospital until we threatened a lawsuit if they didn't get her off the vent now. She was off that vent in the morning and is still alive today. It seems venting is the first thing they do now.

This is a discussion for the doctor and the family.
And this is your MIL.
Are you her POA?
Are you the one that has to make this decision?
If you are not POA and she has family they are the ones that make the decision. Your "moral obligation" is to support your spouse but do share your opinion.
Do you know what her feelings have been on this?

Personally I would not have wanted a vent if there was no chance that I would come off of it. Depending on other health conditions I probably would have been on Hospice and died peacefully at home.