There's a useful thread on GBL/GHB addiction found here in EADD. I've seen it first hand and the withdrawal is definitely alcohol/benzo-esque and very unpleasant.. the key is to avoid 24hr dosing or dosing to get back to sleep if you suffer from rebound waking, and to not dose constantly throughout the day otherwise physical addiction is a very real possibility. I'm not sure about the timescale but I really would avoid using it daily if you want to be safe..