In the 4E PbPs I'm in, generally a post a day is considered the bare minimum. Most make more posts than that, especially outside of combat. Combat isn't problematic so much as simply slower than other parts of the game usually are. RP between characters can go really quickly if the person you're talking to IC is online. Between NPCs it depends on how often your GM is online.
Combat is slower because, at least for 4e, you're bound by initiative order, so you can't act whenever you wish. Because of this you might find yourself waiting for a player to take their turn, or the GM to complete the combat update (where did the enemies move, what actions did they take, etc.) Many players find that a good way to make sure that combat flows well is to plan out their actions somewhat, a good strategy is to PM your GM the possible actions you wish to undertake, given certain happenings, and include the relevant rolls with that PM (we use Invisible Castle for rolls). The worst thing that can happen, then, is that the situation ends up outside of those predictions and you have to wait for that person like you would have had to anyway. Best case the GM says "so and so takes this action, next..."