Not at all times, at least in most cases I've seen a regular 5 minute sync happening for the sandbox instance-org integration. With that being said, although very rare, but at times I've also seen the sync happening at an interval of 15 minutes (or even more) in the sandbox instance-org integration. I understand that during busy/peak load times sandboxes are throttled a little so the production instances aren't adversely affected by the sandbox environments.
Also the sync delays are also caused when there are larger # of updates being synced b/w Marketo instance and SFDC org, notably in that case even the production integration would also experience some delays in syncing data.