Let's do a recap:
Most of us have the trouble that once you plug in the iPhone to a PC (or mac

), the phone starts to
back up, and it takes way too much time.
Backing up is not the same as
syncing. The way it works is that the phone backs up whatever data it's backing up (the $64,000 question for most of us) and then it syncs phones, music, and whatnot.
In my experience, the syncing doesn't take long at all, just the usual when uploading large video or audio files into the iPhone. But I haven't been patient or lucky enough in the past days as to let a back up to complete.
One of the workarounds is to cancel the backing up process; however, if you do this and you actually need to restore your iPhone (for whatever reason), the backup will not work since it never actually completed, so be aware of that.
Some say that the backing up process takes long because if the phone crashes (for me it's daily), it sends a report to Apple once you plug it into your PC. I do think this is right, however, maybe the iPhone sends
the whole phone backup to Apple in order for engineers to replicate the problem instead of just a report. This is what
I think the reason is for the long "back ups". Just yesterday my phone crashed and today it was taking more than 20 minutes to back up and it wasn't nearly half completed.
I will try erasing the current backup and letting a new backup complete, to see what happens, but so far, no full answer or fix. Sorry

.