Just like I did for mid-level 3e, I estimate 1 hour per major encounter.
Low-level combat can take quite a bit longer in 4e, while high-level combat is generally faster.
Solo fights almost always drag on (ha!) longer than you'd expect. Fights with an Elite opponent can take a long time if the Elite's defenses are high.
There are huge variables involved with this, though... If your group has more strikers, combat will be shorter. If it has more leaders, combat will be longer (because there's less incentive to quickly finish off enemies with Dailies). If your team works well together, combat will go much more quickly. If your team spreads out their fire among multiple opponents, it will drag.
-O