I could have written your message a year ago as it was what I wanted for my parents. Essentially what a dutiful nearby daughter would do, but that I wasn't able/didn't want to do for myriad reasons. They had been living indepedently until then with a cleaner a couple of times a week.
A year later, DD is an wheelchair with advanced Parkinson's, my mother has just been diagnosed with Alzheimers and they have a live-in carer along with an additional carer to give her a break for three hours a day. This is all through Home Instead and is obviously costing a fortune, but they wouldn't be able to live at home without it.
What I would say is that always aim for something more than you currently need because by the time you organise it/persuade them to accept it, the chances are it'll be the right level or less than you need. The good thing about starting soft (a carer for an hour in the morning and the evening) meant that they got accustomed and by the time we needed to escalate, they were more amenable. Going through an agency meant that this was something we could do through them.
Sadly once the deterioration starts, it only goes one way and it tends to accelerate.
On a positive note, previous to the live in carers my dad had ended up in hospital four times in eight months. In the five months since we've had live-in, he hasn't had to go to hospital. My life is immeasurably easier as I kept on having to drop everything to travel the two hours to help (with a teenager with her own issues at home).