When a new lead is created in Marketo via any source other than SFDC then it will remain in Marketo until it is explicitely told to sync to SFDC. You can sync the lead manually via a single flow action or best practise is to have a series of trigger campaign set up so that when a lead fits a certain criteria it is synced to SFDC.
For example, if the lead score is greater than 50, if the lead status is Marketing Qualified, if the lead fills out a specific form(s).
Once a lead is pushed to SFDC for the 1st time it will remain in sync with the mathcing SFDC record until one of them is deleted. Similarly if a lead is created first in SFDC it will sync down to Marketo and remain in sync until one is deleted.