It shouldn’t take longer than 5 or so minutes for the backup to complete though it all depends on how much data is on your device. iExplorer is making an iTunes backup at that moment. If it seems stuck on making a backup (taking longer than 10 or 15 minutes), please close and reopen iExplorer and right-click on Backup in the left panel and choose to Back Up Now which will make a new iTunes backup. If the backup still doesn’t finish or load, please see this article:
Why is my Backup data not appearing?
Then if you still have any issues, please submit a support ticket.