Bob, the thinset bag has a list of times on it. Working time, pot life, etc. Pot life is how long it stays good in the bucket. Working time is how long you have to get the tile in it's final position before you mess up the bond. These times are considered absolute maximums, so try to leave yourselve some margin.
The bag also says how much water to mix. It may tell you how much to mix for 50 pounds, though. You'll have to measure the thinset by weight and add the correct proportion of water. After you do a couple hundred bags, you'll be able to estimate it by eye, but for now, weigh and measure.