It's not polyester, it's polyethylene.
That all depends on the overcast, UV protection the tank has, tank color, the fert. etc...
General rule of thumb is to mix only what you are going to use that day.
That can change some, but I wouldn't mix a full tank and expect it to be good 3 weeks from now.
This stuff does spoil, and the sun will do that fast. It can break it doen real goon sitting in a tank in the sun.