I tend to go to "a lot" of cons, and can do so because I budget well and my city has a large fan-community, which is to say that there is a con or large event of some kind (comics, anime, sci-fi, fantasy, gaming, etc) at least once a month either in the city or a reasonable travel distance away.
This year is 10 cons/large costume events for me - but I also skipped 7 conventions which are local enough that I wouldn't have had to pay anything besides admission to attend. Can't do everything.
As for when it becomes too much? For me it's when i do a lot of conventions consecutively - In July I was at a convention 4 out of 5 weekends and by the last one I was REALLY ready to take a long break and actually have the time/energy to do things like work on costumes.