We all know that it is only possible to get 4 digsites at a time, and in Kalimdor there is a total of 63 digsites, which contains 9 Tol'vir digsites.
Most people tend to farm until they have a reasonable amount of spawns in Uldum, but this is not the most efficient method, here's why:
You have no digsites in Uldum, that gives you a 9/63 ~ 14,28% chance to get one in Uldum.
If you have one in Uldum already, the chance is 8/63 ~ 12,69% chance to get a second spawn in Uldum.
If you have two spawns, the chance is 7/63 ~ 11,11% for a third spawn.
If you have three spawns, the chance is 6/63 ~ 9,52% for a fourth spawn.
What you can see from the calculations above is that the chance of getting a Tol'vir spawn diminishes by the more you already have in Uldum.
This means that when you get a spawn in Uldum, you should go to Uldum as fast as possible and clear the digsite instead of trying to stack them.
Trust me, with the amount of grinding required to get the rare Tol'vir items you want to consider this.
Hope it helps.