How thick can this be put down?

This is a great material. The thicker that you put it down, the longer it will take to harden. I put it down at up to a half inch thick and it was set hard enough to walk on the next day, but it was fully hardened in about 3 days. I would guess that at 3/4 inch, you should allow a week to fully set up. The bad news is that Lowes in my area has decided to stop carrying this product in the gallon size and the stuff that they recommended to take it's place didn't work at all. (They still do carry it in the quart size, but 2 quarts cost about the same as a gallon did.)

DAP® Bondex® Flexible Floor Patch and Leveler can be applied up to ¼ " inch and let dry approximately 6 hours to a hard consistency (depending on temperature and moisture levels). For thickness greater than ¼ " inch, apply in multiple coats allowing each coat to dry completely between application to the final desired thickness.