Depends on the way you set them up and how often you refresh/remake the cultures, I'm pretty lazy with mine and they do end up stinking, but they are very hardy little things and you will often find them still alive after months of neglect (I know because I looked at mine this morning for the first time in a good 3 months and they were still squirming away).
As per the success rates, how often are you doing water changes? Good clean water is a vital ingrediant to growing fry up well.