Basically what Brovo... said? 

It really varies RP to RP, and even say most people (GM including) drop out, it can still go on if those remaining keep it moving.
For one fun example, very first RP I joined on the Guild me and two others essentially went post crazy. Covering about 10 to 20 pages a day, this caused the majority of players go "I can't keep up with this" and quit, which by some people's definitions (X people leave the RP) would cause it to be 'dead', but in truth it was the exact opposite, it was so full of life that some people just couldn't handle it.

In terms of the other commonly foreseen death of an RP: "X amount of time with a post" I'm having trouble thinking of a specific forum example atm oddly enough. :/ However this one is easily countered regardless, people have lives/other plans. Something might of happened that caused the RP is take a break because maybe they just moved, a relative died, they just had a kid/got custody of their kid etc. Stuff that would take them a few months to focus on and adapt to before they could RP again, and assuming the players are still in touch they can easily just start it from where they left off.