If they are "steeping grains", that is crystal type grains, you'll get nothing more out of them than they can give so 15-20 minutes is fine. Steeping it for longer won't give you more flavor or color or sugar out of them.
If they are "mashing grains"- that is base malt, or grains that must be mashed, they should be in the liquid for 45 minutes to an hour so that the starches can be converted into fermentable sugars.
Steeping grains is like making tea- for the first few minutes you'll get more out of them just like when you make tea. But steeping tea for 30 minutes won't give you better tea than when you steep for 10 minutes. The same is true with steeping grains. Once they are "done", they are done.
Broken Leg Brewery
Giving beer a leg to stand on since 2006