Really just depends on your climate...however you should get at least 2+ months out of an application based on what I've experienced using it on a DD here in AZ weather.
I've gone 4 months between applications and it was still beading well.
$80 on the gallon is a bargain just as Dylan said. I usually tend to split my gallons with a friend if I feel they're going to last me way longer than a year or two.