Also remember that taking 10 or taking 20 takes 10 or 20 times as long. So thats fine for him to do if there is no time restrictions, but if there are any at all then he will need to choose specific locations only that he will take 10/20 on.
If a rogue is taking 10 move silently, hide and searching, then he is going to need about 2 mins per 5 foot square (I say 2 because it probably shouldn't take any more time to take 10 on MS and hide in the same instance, but if there is real risk of being caught, such as searching the perimeter of a bugbear camp in the forest with inches of dry leaves and sticks on the ground, then it would be safe to say 3 mins per square).
This means he is going to be covering about 20 squares per hour and if I was DMing I would be rolling for random encounters every 1/2 hr to hr, and roleplaying out every square (describing anomallies on the floor, the color of the stone, suspicious cracks that he soon realises are innocuous, etc) until the rest of the party if not the PC in question, get bored and force them to just roll for each square. The only exception is if they have totally cleared out a crypt or the like and while the rest of the party is taking a day off to rest there, the rogue wants to throughly search the place. Then I would just let them take 10 or 20 and be done with it, only roleplaying out places where they actually find something amiss.