Loremaster Belannaer was considered to be young when he first entered the White Tower during the rule of Bel-Hator now he is looked upon as rather old. Since we know from the 6th edition Dark elf book that 100 years of age is around the time that high elves join the militia, and that this is considered to be very young, we can expect Belannaer to have been at least 300 at the time he entered the Tower (considering that fact that he, being a prince, probably served a couple of centuries in the Silver Helms). This puts Belannaers age at everything from around 1350 to ca. 650. I would guess that Belannaer still have a few centuries left in him, he certainly does not seem to fade in any of the fluff we are served. If pressed I would say that elves probably live from ca 1000-2000 years but some are capable of living considerably longer if they feel they have a reason to do so.