I run phosguard too and I change it about every other water change (so about 3 weeks), it just depends on how "hard" it is having to work. If you have large amounts of silicates or phosphates then every few days, if not, then it all depends on bioload. Some people do it every week, some once a month.
Try changing it every two weeks, if it is still keeping your levels down where they should be then try three weeks, and so on until you get to where your parameters are rising before you change it out and you know that is too long, so you know to change it a week sooner. Just need to find a balance. Am I making sense? Haha
The hardest thing about this hobby, is having a budget..
Everyone has questions