Hi Clinton,
By default the sync interval in a Marketo is set to 5 minutes. This can be set to be greater than 5 minutes on request.
Now this doesn't mean that Marketo will sync updates every 5 minutes, it means that there will be approximately a 5 minute wait after one sync cycle finishes and and the next sync cycle begins.
Divya is right, the duration that an individual sync cycle will take depends on how many records for each object need to be updated/created. (That's also my favourite marketo community article!)
Remember flow actions in smart campaigns are immidiate, so if you ever don't want to wait for the background sync for a specific lead or set of leads then you could always run an immidate flow action, just bear in mind that flow actions use 1 api call per lead record whereas the background sync is much more efficient and can handle 200 records with just one api call.
Rgs,
Cathal