How long do these attacks by the Dragon Priests typically last(I'm assuming it's a regular scripted event, due to the NPC's reaction to it)? I was traveling through on my Empath and got sniped and the guard won't open the gate to let me through because the city is under attack by Dragon Priests and his mid 20s combats aren't doing much good.

Only the gear gate is barred during one of those invasions, so its still entirely possible to run around to the other gate if you need to get into Raven's Point itself, or just simply bypass Raven's Point entirely if you are just passing through.

The route is marked on Ranik's maps fairly clearly.

-Evran

Moongate, Ressurection, and Fire Rain, Oh my!