How often do i need to sync my iphone?

iPhone is designed to be synced on a regular basis. First thing done when you sync is an iPhone backup. Some folks sync daily, others it seems never sync. I'd suggest syncing anytime you want to insure your data is backed up or no less than once a week. If something were to happen to your phone, like your computer, you want to maintain a current backup to restore from, should the need ever arise.

Theoretically, you could sync it once and never again. There is no set number, but I would do syncs more often, because every time you sync it, it does a backup. And if it is broken, you can restore your new one directly from the backup, and all your settings, gamestats and music is there as it was before.

Personally, I try to sync daily, mostly to automatically pick up new podcasts, and app updates (which I prefer to download to iTunes on my MBP. I would also suggest that you sync sometime soon after you purchase a new app or a tune on your iPhone - for the backup benefits mentioned by others. It means restoring your iPhone in the event of a serious problem is much easier.

As already said, often enough so if the phone crashes you won't curse your luck in not having a backup.

It is just as important to back up your computer; we get several messages a day from iPhone owners whose computers have crashed and they have no backup of the iPhone content.