Most pros consider the pre-mixed stuff junk and expensive. It stays soft in the bucket because it needs to dry out to do anything, and can re-emulsify if it gets wet again. It will take a LONG time to set that thick, under a porcelain tile, months. You'd be much better off with some real thinset for this and to set the tile. Note, though, you should do this is a couple of layers since it really is meant to be THINset. An alternative would be to buy a bag of medium set (often called granite and marble) mortar...it has more sand in it and can be installed up to about 1/2" thick (thus, the medium set as opposed to thinset). That can also be used to set whatever tile you want.