OmniFocus doesn't use OmniPresence, specifically. It uses the Omni Sync Server (which OmniPresence also saves to).
OmniFocus syncs with the Omni Sync Server I think hourly, unless you make a change, and then it syncs after something like a minute. It's been explained in the OmniFocus forums; given this question is for OmniFocus and not OmniPresence (which can be used by OmniOutliner, OmniGraffle, etc.) it's probably more appropriate there.
OmniPresence syncs with OmniOutliner as soon as you hit the "save" button in OmniOutliner
