Also, you have to consider that SQL will stop a backup even if there are open transactions. I know because we have had a rogue developer who was given too much access by the CIO at my last job who more than once left a transaction open by mistake for days, but no one caught it at first because he wasn't hitting recent data (he was fixing old data). But we didn't get backup failures or backups going longer than normal, so SQL Server obviously at some point ignores open transactions and finishes the backup.