Hi Michael,
Unless that lead is being told to sync immediately by a smart campaign which uses a Salesforce related flow step (like "Sync Lead to SFDC"), updates like the one you describe will occur in the background during each sync cycle. Marketo reaches out to your Salesforce and checks all of the different objects (Lead, Contact, Account), pushes updates from Marketo, and pulls down updates from Salesforce. Once it's complete, it waits 5 minutes and then starts again.
So I would always give it at least 5 minutes. If there are mass updates happening (suppose you're changing values for a lot of accounts), it could take significantly longer (hours, even). But in general it should just take 5 to 15 minutes.