Unless iPhone is syncing with your computer, you can disconnect iPhone from your computer at any time.
When iPhone is syncing with your computer, iPhone shows "Sync in progress." If you disconnect iPhone before it's done syncing, some data may not have been transferred. When iPhone is done syncing, iTunes shows "iPhone sync is complete."
To cancel a sync so you can disconnect iPhone, drag the slider on iPhone.
Tip: If you get a call during a sync, the sync is cancelled automatically and you can unplug iPhone to answer the call. Connect iPhone after the call to finish syncing.