The escort for the healers quest has about a 10 game-day limit, but as long as you head in the general direction of the location that will be marked on your map, you should have lots of time.
There are a couple portals which will disappear if you do not go through them within some time (one on the first level in the catacombs, one in Nericon's garden). There can be problems joining the thieves' guild if you accept either of the two quests, but do not complete them in a timely fashion.
