I would say the ideal time to look at care homes, is before a crisis, so that you have plenty of time to assess the options and deal with paperwork/financial/legal issues.
As to when it's time to make the move to a CH, well, that's difficult to say, and impossible to predict. There could be a health issue or medical problem or accident, at any time, that might require some sort of institutional care (hospital, care home, rehabilitation, whatever) and of course you can't predict that. Or something might happen to you, to make it impossible for you to provide care. Things happen.
If only the dementia diagnosis came with a crystal ball!!
So probably the best thing you can do is to make the best decision you can, with the information you have, right now. I'm sorry if that sounds obvious but what else can you do?
Having said that, is there any information you could get from any health or medical professional, or social worker, or any source, to give you more insight about current needs and even a general idea of how things might develop? It probably can't hurt to ask.
I hope you're able to make a decision that works for you.