I used that stuff, and correctly mixed, I got probably 1.5-2 hours out of it. To mix the stuff, you need to add the powder to the water. It really helps to use a drill with a mixing paddle. Trying to do it with a trowel is possible, but tough. Once you get it all mixed up, the instructions say to let it slake, or sit for about ten minutes, then remix. That gives the stuff a chance to absorb all of the water, then mixing again should produce a nice smooth mixture. If it starts to get stiff, you can remix probably at least once before you have to throw the stuff away.
A really dry and hot location could make the stuff start to cure faster.
Longtime lurker, avid DIY'er