It depends what time you'll be doing the journey and where in Bristol you're aiming for.
It takes me about 40 minutes to get from near Chalford to central Bristol, but I leave at about 6.30am. Much later and it's close to an hour, and if you hit the M32 after 8-8.30 it can be a lot worse. Coming home is about 50 minutes.
It really does depend where in Bristol you're aiming for though.
You have the choice of A46, M4, M32 or you can go down the M5, M4, M32 (I do this in then come home up the A46).
There's the A38 too, and various ways to cut through to the A46 (or M5).
If you cut through Avening/Tetbury and go past Westonbirt to join the A46 it isn't to bad a journey. And it is a pretty commute for the most part.
Horsley would be straight out onto the A46, but not too far to head out towards Wotton under Edge and the M5/A38. Again, depending which part of Bristol you want.
Google maps makes it about 10 minutes longer from Chalford to my office.