Fugazi,
Clean your filter based on the pressure drop increase not a certain amount of time. One of the biggest problems with cartridge filters is over-cleaning. They actually work better with a little debris on their surface. Most manufactures recommend cleaning when the pressure increases bu 8-10 psi from clean. I prefer to clean when the pressure increases by 20 %. There are so many variables that affect filter clean intervals you just can't do it by time interval. Things like dust levels, covered or not, pool use frequency, tree coverage, and filter size to name a few. I have a pool a little smaller than yours but it's covered with a screen enclosure and we have a 200 sq ft filter in it. I clean it 2 times per year using one of those filter comb attachments to my hose.
An easy way to keep tabs on your filter capacity is to get one of those gauges that you can reset the bezel to line up with new pressure. Then just watch the needle rise to the "clean filter" mark.
I hope this helps.
Chris