Butterflies have a one-century grace period in Ixbiliada.
A Point of Divergence in 1081 is just this side of plausible for the survival of al-Andalus (at least in its native form, rather than as an extension of a Berber empire.) It also has some unfortunate side effects, like butterflying a number of recognizable names out of existence - St. Dominic, Averroes, the Rambam, you get the idea - and by rights this should make the world completely unrecognizable. (Which is especially serious because it means that the more important works of the Convivencia period - the Mishneh Torah, the Guide to the Perplexed, the Incoherence of the Incoherence, etc. - can't be assumed.)
My rule of thumb is, thus, that (within Iberia itself, at least) there's a 100-year grace period in which intellectually important people can still be born. Everybody else has a (100-years from PoD)% chance of being butterflied away, unless there's reasons to make that chance lower sooner.
It's worth noting, though, that the changes are going to start earlier; even if Fame Equals Birth, it doesn't equal an exactly identical history. I'm going to gloss these changes because this is an alternate history, not a character study; ibn Bajjah's entire life is going to be colored by the Aragonese conquest of Zaragoza around the time he was born, but is that going to change the nature of his writing? Probably not.
Once I get out of the Middle Ages, all bets are off.