Yes, that is always implied. Even Guido, the original BFDL, stepped down. Linus will probably retire at some point (I hope, considering the alternative would be death before retirement at a relatively young age). Etc.
Which is exactly what happened to the first "dictator for life" (without the B) ;-). Even at more or less the exact age where one would expect someone to retire today.
See also: Julius Caesar, Ides of March
Edit: Just to make this clear, I also find the "I'm doing this as long as it works out and makes sense" to be the only useful approach to the "title" of BDFL.