Go into "Settings" and then "Display". The sleep option should be in there. Then just change it to the amount of time you want it to stay lit.
But, a word of warning... The longer you leave it on, the faster your battery will drain. So I would really suggest only setting it for longer times when you are actually needing it to be longer. Then go back into "Settings" and change it to a shorter time frame (15 or 30 seconds) when you don't need it to stay on. Also remember that many apps, as long as you are actively using them/touching the screen will keep the display active, without changing the sleep time.