According to the Wiki, you can expect approximately 54k experience per hour in Thieving at level 51. That will, of course, vary based on luck opening doors and urns as well as any experience-boosting items you have such as the clan avatar or Black Ibis pieces.
“Eragon looked back at him, confused. "I don't understand."
Of course you don't," said Brom impatiently. "That's why I'm teaching you and not the other way around.”
If I remember correctly, you should be able to get up to 60K XP per hour at level 51 if you loot urns in room 3 until the timer is half up and then move to room 4. It's very decent XP, as you could reach level 61 relatively quickly (when XP rates increase again).