Depends on what you're doing, how many cycles you've done previously, the brand you're using, etc.
I and other "newbies" got good results from using only 15mg/day (5mg 3x day). Do a search and you can find posts.
If your goal is to diet without losing muscle, then you may be able to get away with only 10mg-15mg/day.
The "problem" with ox is that it's one of those roids where more is better and the longer cycles tend to work better. After 3 or 4 weeks on 15mg I bumped it up to 20mg and there was noticable improvement...then I wanted to bump it up to 25mg... Same thing with cycle length: I'd really like to get about 10 bottles of this stuff and just run a cycle out for months and months until I see the results stop.
To directly answer your question though, I'd recommend 1/10th your bodyweight per day (200 lbs = 20mg/day) as a minimum, and plan on running 10-12 weeks.
p.s. Ox totally messed up my cholesterol profile. Just an FYI.