Maybe a week if you store it in the frig. It helps if you used boiled water when you make up the solution.

I also mix the Sta Flo one to one (1 c water to 1c Sf) as I like my fabrics to be stiff. Also if I have several pieces that need starching I use a hand held Garden Sprayer and lay my pieces on a sheet and then just spray away and when done I also toss my pieces in my dryer to dry and then I use a light sprinkle to press the fabrics.

I've left mine much longer than a week and its been fine. I may have to clean the nozzle, but I don't refill til I need to. I also throw in the dryer for a bit to speed the process.