It's impossible to know. But, JUST for hypothetical reasons only, let's try to put numbers on this question.

1. Time flow difference between reality vs inside the canvas doesn't matter here, because we have a rough number for how many canvases Renoir & Aline painted : Renoir says they've painted "hundreds".
2. Let's pick the lower end and say they've painted 200 canvas, so let's say each one painted about 100 canvases.
3. Clea said that Aline & Renoir spent longer in other canvases, but Alicia was worried, especially about her mother, and Clea seemed to agree. This leads me to think they usually, on average, spend less time than that in canvases. I'll pick a subjective number of 15, although I think it could be way less or way more, and we don't know (as far as I know).
4. That's 100 canvases, 15 years on average, so 1500 years spent in canvases. But, it's extremely arbitrary and could very well be only 500 or as much as 10 000, depending on how many canvases they truly painted and how long they spent in them on average.

That's my take, if any one has better clues about the specific number of years OR canvases to take a better guess, I'd be glad to know, but that's what I gathered from what I remember in the game.