At, say 1/8 inch thickness, how many square feet will this cover?

Don't know the math, but I have used 3 1/2 buckets for 3 1/2 walls that is 8 ft by 5 ft plus the ceiling.

Ya might get about a 8' x 8' area. It all depends on your actual finished thickness. It also depends on your texture build-up (how high +/-). That is probably the furthest you'll get from that size bucket. Best of luck... Ya might also need to prime (concrete/plaster type) to make sure it sticks too.

How much it will cover will depend on if you are using it for the texturing. For 1/8" coating you can guesstimate approximately 80 square feet.