As many as you can get in there. As far as leaving, all some one has do to is check PH daily and maybe adjust it. You can setup a your unit with fresh water and nutes, PH it and leave for 2 weeks. If they can move a light they can top off the res and check PH. They might have to adjust PH. Just check it and add a little up or down depends on how much water is in the system, a teaspoon to a tablespoon, wait 15 minutes and check again. Buy PH up and down when you get your nutes. If your Ph is good they won't have much to do. Changing res water is the only real work mine takes. The rest is just checking and alittle adjustment. You should have no problems. It's good you have some one you can trust. Check out what latewood said you may save some you can spend on a good PH meter.