I just keep playing until I realize that I'm not really having fun. I try to avoid characters that aren't much fun.

As far as longevity goes, I try to set some goals for myself:

#1 Get to hero. This can take anywhere from 40-150 hours typically. Getting there can be fun, but tedious.
#2 Have an RP goal to accomplish on your own. Typically, I think of mine by level 20 or so and don't put it into full-gear until I reach goal #1. But then, I tend to powerrank.
#3 Involve others in your RP, even if it slightly changes the goal. No woman is an island unto herself. There are other players in CF and, as a whole, many of them just kinda refuse to RP with one another. Sure, you might have a conversation every now and then, but when was the last time you RP'd an exploratory quest with others? (i.e. Help me find my family!)
#4 Set up personal challenges for yourself. What was your previous best PK ratio, and can you beat it? How many hours can you go without dying?
#5 If worse comes to worse and you're dreadfully bored but you can't quite motivate yourself to type delete/delete, try a complete change of pace. Put into motion something no one has done before. Try to get your drow turned goodly and join the Fortress. Try to get your felar turned into a shaman. People learn and grow, so you could make this viable from an RP perspective. *Note* This is VERY hard to do, though, and expect it to take a long, long time.