I wish I got two weeks out of my filter socks JK47!! The 100micron ones on my 1400L lasted three days before they needed changing, I got so pissed off with the regular maintenance I'm almost put off using them again and looking for alternatives.
As for the original question, I wouldn't buy new ones every time, but it would be good to have a second pair so you can just swap them out when they get dirty and get the tank running again while you clean the dirty pair out. You can change them as often as you want, the more frequently you remove physical waste from your system the better, but generally you can wait until they look like they're getting close to overflowing.