How long do have to use the mixed compound before it starts drying up?

It depends on the temperature of the water you mix it with greatly, colder water will give you more time than warm or room temp for sure, as a rule i usually figure half the number on the bag, once you succeed half the number the product gets firmer and firmer until its too hard to work. This half rule is meant for cold tap water, like I said warmer water or if your bucket of water has been sitting in the sun all day and warm, you will notice the mud setting faster.

The product has a 45 minute set up time.

Quick-Set will harden in the time stated on the bag. Quick-Set is available in 5, 20, 45, 90, 210 minute formulations. Allow 24 hours to completely dry before painting.