Regarding your timing, what I did was take a metronome at a slow tempo and only play one note per click and gradually speed up from there. Also, 3 string sweeps would be good for a starting point but I don't think I'd condemn working with more. Just get comfortable with what you can handle before getting carried away. Regarding how much time do we practice sweeps, well, that differs between us all. I do a lot of sweeping so I got reasonably good ad an adequate pace, but compared to the guy next to me who's doing fewer sweeps , it just depends, but you'll get out of it what you put in.