You only have a minute or two before it starts to set up...depending on the amount of hardener (the red stuff) you use. Just have your area cleaned and ready to work.
Try a test patch somewhere first. Mix it up until it turns pink all the way through and practice spreading. You will get the hang of it fast enough.
You can use it over the screws. But to be honest, using simple spackling is the best way to cover the screw areas. Easier to work with, less fuss and dust, and once painted you would never know.
Keep a couple of those old magnets on hand. You can use them to find the screw areas if you need to take it apart. This is why spackling works great because it is easier to break through it to get to the screw than bondo.