When it comes right down to it, the important factor is "how fast do you want PCs to level?" I'd say 1 PC level per 2 or 3 gaming sessions is about right. Since there are about 10 encounters per level, that means 3 - 5 encounters per session.Since we only play twice a month, I don't really want to make it go slower; ....
In the 4e group I ran a while ago, the PCs could go as long as 7 encounters before an extended rest. ...But the players were good tacticians; I imagine less skilled players might only get through 3 encounters before a long rest.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.