3/32"
It all depends on how leveled the walls or floors are. If the floors are not leveled, you can have minimum thinset on one end and 2" on the other end of the floor. The same applies for the walls. To make the job easy and to use minimal thinset, it very important that the subfloor/ mud job is leveled and the the walls are framed out right. Some contractors would instead use a lot of thinset to leveled out their surfaces which is not the right thing to do; basically they are thinking that they are cutting a step in the process but instead end up paying more for thinset.
The size and thickness of your tiles will determine the thickness of your thinset application. A large stone tile could use a 1" notch trowel where a smaller ceramic shower tile could go down to an 1/8th" notch trowel. Every tile is different so there is no standard thickness.