We recently updated a HUGE amount of data in SFDC (specifically, the Account Type field) and have yet to see the change take effect in Marketo.
I read in a help article that it sometimes takes a bit longer for the data to sync to my Marketo instance, though it's been 48 hours and my lists are still pulling outdated information. It's very likely that I'm just being impatient, but I'm wondering if anyone else has experienced this and has an idea of how long it will take for this information to port over.
I double checked the sfdc sync in my admin tab and all is well there, with the most recent sync happening at 8:44am EST this morning, and the field mapping is set up properly, with no changes having been made recently.
Re: Salesforce syncing after a large data overhaul
If I'm understanding correctly, you're attempting to write to a SFDC Account field. Out of the box, Marketo can only read SFDC Account fields so even if the change is made in Marketo, the changes will not be reflected in SFDC.