So what year does "MoTU" take place in the FR timeline?

[Monkey-Boy]

Has this ever been established or has it been left vague on purpose?

[Krelar]

Has this ever been established or has it been left vague on purpose?

Looking here: (http://forgottenrealms.wikia.com/wiki/Lolth)


Post-Spellplague
At some point in the years leading up to 1479 DR, Lolth ordered her servants to begin collecting blue fire items and relics important to Mystra. Her ultimate goal was to become the new goddess of magic.

This sounds very similar to what is going on in MoTU.

[GoldyGopher]

There was a vague answer given...

Mystra returns in 1479 DR, Caught in the Web takes place slightly before that (1475ish DR)

The other quests in the chain take place a few years earlier. (1470 to 1475ish DR.)