I have always liked using Custom Building Products fast-setting thinset.

Years ago, Custom Products Speedset would harden way too quickly and would start setting up in 20-30 minutes. They have since adjusted their formula to allow for a bit more open time. They claim 4 hours working time in the bucket but I would not allow this much time. I would not mix more than you can put down in one hour. Also, once the thinset starts to harden in the bucket the worst thing you can do is to add water to soften it. At that point just let it go and mix up a new batch. Let me know if you have any specific questions that I have not addressed. Good luck!