On Mother Gothel's wiki page, it says she is centuries old.
Spoiler ---
At the end of the film she dies of old age so I would say over 400. Hope it helped!! :)
If you want to know how old she can seem to be, I can answer she looks like a 50 years old lady. But we know from the story that she used the flower for centuries, as you have said. My opinion. Good question.