I think it's meant to vary by encounter, sometimes I'll do 4-5 encounters before needing to rest, using only a couple spell slots/LR abilities each time, sometimes I get absolutely savaged and need to rest after only one or two. /shurg It does get easier as you level, and long rest casters in particular get more resources. Worth noting too, the time-sensitivity of the quest becomes... much less so, shortly. :P Still urgent, but not "you have one week, then you die, and the Absolute probably takes over the world" urgent.

I suspect the game is balanced around the idea you'll probably do 2-4 encounters per long rest though, purely given the ratio of short to long rests.