I believe for phosguard you're only supposed to leave it in place until it's fixed your problem. It's not like
GFO. Ideally you run phosguard for 4 days and test your levels. If you still need to lower levels, run another 4 days.
It's also better to use less of at once until it's completely exhausted. Using more is not more effective with this particular product.
Since phosphate is a necessary nutrient for all living organisms I'd run it no longer than 8 day and 12 being the most unless my test kit said above .03 phosphate (and even then I wouldn't run it longer, I'd figure out how my phosphate levels are not decreasing and figure out what I'm doing to add phosphate or what is disturbing my sand bed)