I ran a 40 minute A main last weekend and my filter is still clean...
Man those ebay cover things work good! They have a special running right now on the yellow Xray, RB, Mugen size one for $5.99 or something like that. They are cheap work well and I can run this filter for a whole practice session saving some cash.
...got mine here:
http://stores.ebay.com/UndergroundRCParts