A lot depends on what club, and where you are at...i/e fairway rough for approach? Green side rough, how deep/etc., how far off the green are you?..green speeds and elevation are also something to consider as well.
I/e, there's no set "add X yards" to all clubs, IMO, it scales..i/e in 20/25%rough, with PW, I just adjust for topspin/backspin on the 115 yd club.
But see as much 15-20 yards with 3 iron in same rough.
As much as I like BB's advice usually, I chip from 30/40% rough most of the time, especially on BPB #5 and #9 if I'm off the green, but usually #5 leaves me 18-20yards from hole, so I pitch.
The 40/50 greenside stuff I use a pitch, even if 8 yards from hole.
You really have to find out what you're comfortable with, I fell in love with the flop around the green, from fringe (especially) to 30/40, but over the past 3/4 weeks, chip is becoming my standard shot if I really want to try and make it.