About every 30 mins, usually a bit less. I've walked up to the bus line, finding it empty, and just walked right onto the bus...I was the last to board, and we departed almost immediately. Other times, I've joined the line, waited for 20 mins. (the bus had probably left with 5 mins of my arrival), then boarded the bus, but had to wait a bit before the bus departed.
Figure on it taking about 75 mins, from gate to resort. Some times will be less, others longer.