For me, about 4 years seems to be the magic number for groups that make it past one year.
-I spent about 4 years playing for the church I joined when I moved to Chicago for college. After getting married I moved to a new neighborhood and a new church.
-4 years playing for my first original rock group Something Ordinary. I somehow convinced myself that they were destined for greater things than I could commit to so I bowed out. They never found a replacement and disbanded shortly thereafter.
-4 years playing for the country act Janesville. Things were just winding down. They also folded shortly after I left.
There have been a few short-term projects in the intervening 6 years but I finally joined an actively gigging singer so we'll see how long this lasts.