Normally, a hefeweizen doesn't even need to be secondaried but, on the other hand, your OG is way high for a hefe and it's usually recommended that high-gravity beers be secondaried so it's pretty much your choice whether you do or not. In any case, you're not really shooting for a particular final gravity. Rather, once you think it should be done, you want to check the gravity reading daily. Once you get the same gravity reading a couple days in a row, it's done. If you're going to secondary, then is the time to transfer. Or, if you're going to bottle, you can do it then also. The important thing is the fermentation be complete.