I hook my kegs at 30-40psi (and sometimes add gelatine) as soon as I put them in the fridge and leave there for 48-60 hours depending on much carbonation I want then dial it down to 8ish for serving.
I like my IPA's quite carbonated (I aim for 2.6 volume) but have no way of measuring what the actual volume is... I use the chart bellow as a reference and go by "feel"... That seems to work quite well.
The warmer the beer, the longer it takes... I've carbed a couple kegs outside the fridge (around 55F?) and I it took about 10 days to get a decent carbonation on them.