As much as possible is the short answer. The question presupposes that domestic cats are intrinsically independent and solitary requiring a controlled amount of human company. That’s wrong. You can spend all day, every day with your cat and she’d be very happy provided you treat her well and are good company.
I spend all day and night with my cat except for about 2 hours daily when I do outside for chores or exercise. The longer you spend with your cat the closer the bond and the happier your cat will be.
That does not mean that you have to be all over your cat like an octopus. It means that you let her dictate how close you’re going to be in terms of physical contact. Let the cat decide.
Some want to be in contact with you while others want to be in the same area but on their favourite perch or bed looking out the window. Let them express their individual characters as it will also help them to be content.
Domestic cats enjoy being in the same space as their human companion provided as usual that the human companion is a good one. This means a person who is sensitive to the demands of their cat; their likes and dislikes. Sensitivity to feline behaviour leads to a great relationship. This means understanding cat behaviour. Cats don’t need to be left alone to do their own thing.
Domestic cats are no longer solitary. They are essentially social creatures after about 10,000 years of domestication. However, they need and deserve to be living with decent people who respect them as cats.
The flip side of the coin to the question in the title is, ‘How much time is too little for cats in terms of human contact?”
It is possible to provide an answer to that question. If, because of work demands or other distractions, a cat caretaker/guardian can only spend about 2 hours every 24 with their cat it is too little. There is little point adopting a cat if all you can give is that kind of limited caretaking.
The cat is likely to be lonely and may suffer from anxieties. And if they are full-time indoor cats they definitely will suffer mental health problems in my opinion. These may be seen as physical problems such as urinary tract disease.
I realise that a person’s circumstances change and the changes may be unforeseen so it may happen that a good cat owner can’t be with their cat for a reasonable amount of time but when adopting a cat it is sensible to ask if it is likely that you’ll be able to dedicate sufficient time to your cat. Any doubts don’t do it.
So there is a minimum amount of time that one should spend with a domestic cat within the 24 hour time frame but not a maximum. There is no hard and fast rule but I’d expect the minimum to around 3 hours awake time daily.
Night-time can count to a certain extent because cats often sleep on the bed with their humans. Let them please. Don’t lock them out of the bedroom particularly if you are away all day working and you are the only person in their lives. The human bed is a great place for a cat in terms of being comforted and reassured because it is full of human scent.
You may believe that you can’t sleep with your cat on the bed with you. You will as you’ll get used to it.