That depends on your clearing speed. I can do GRs ~35-40 in about 3 minutes, add a minute for salvaging/portal closing etc. and it takes 4 minutes for a run. Lets say you get 3 Legs per run (normally its even more) and 0.5 per gambling with 200 shards each run (every second run a leg is more than realistic).
In 1 Hour that would be 15 Grifts. Per rift its about 3,5 legs - 15*3,5 = 52,5legs/hour.
Dashing Strike Monk is the way to go, but even if I go with my barb its about 35 legs/hour.