Not that often and most of the time the 25% only applies to few specific pre-orders. Universal 25% off of pre-orders is somewhat rare and 30% almost unique.
The discount may not always be 25%, but there's at least one discount code per month that will work on select pre-orders. I hardly meant to imply that such codes work on every pre-order -- 2K in particular typically doesn't allow them, for example.
Edit: Here's how the past six months, excluding August, have fared:
July: Up to $12 in GMG credit (effectively -24%)
June: -25%
May: -25%
April: -25%
March: -25%
February: -25%
I didn't do a lot of digging so I may have missed a 30% discount code here or there.