In the meantime you can use skimmer socks in your skimmer and a little bit of DE powder into the filter via the skimmer (take out the skimmer sock first). You want to add JUST enough to raise your pressure 1psi on the gauge. It may be between 1/4 cup and a full cup of DE powder to do that. Get a 10# box--that and the skimmer socks should set you back a whopping $15. The DE should last two or 3 seasons and, if you rinse them out and reuse them, the skimmer socks can last a month each (sometimes called Filter Savers, but just a different brand). Both of these things can really help your filter's performance. Also, as your filter and sand get older and the sand gets "dirtier" it will filter better. You should never change your sand unless your have a really big problem like goop in the filter (from a bacquacil conversion) or a very VERY rare algae infestation in the filter itself--I've only seen one of those.

If,however, you maintain your pool and don't neglect it, your sand should last the life of your filter--and who knows how long that will be!

Carl