Ok say your watch has power reserve of 40 hours if you fully wind it you could put it down and around 40 hours later it should still be running. The auto winding bit is a tad more complicated and depends on your life style, if you are a still life model it's not going to get wound up much, if you are a marathon runner it will get wound more.
So from a full wind you wear the watch one day but only put 4 hours of reserve back into it you will be down to 36 hours, you then go to bed and you then lose another 8 hours, now down to 28 hours you repeat that another day and your down to 24 hours, eventually you will have to manually wind it.
IF however your lifestyle is active enough to put 8 hours of power back into it a day it will effectively run for ever with no manual winding.
I trust that was helpful enough.