There is no minimum amount of time for the items to be in the water.
There should be 60 times the volume of the utensils.
Any material than can be kashered can be kashered this way, however, this will not work for frying pans or grills, which became non-kosher over the fire.
Furthermore, the first rule of kashering utensils is to wait 24 hours after they became non-kosher. This is because waiting 24 hours allows the "flavor" of the non-kosher food to become not tasty. This makes my rule No. 2 less important. (It is still nice, but not essential under normal circumstances.)
If, however, you for some reason cannot wait, you must make sure that there are 60 times the volume of the utensil, and/or you could use a fairly large amount of powerful soap, but in such a case the specifics become very important and you should ask the specific question to someone who can understand the situation.