StarSan remains active as long as its PH is below 3. the kind of water and how its stored will affect how long it stays active. get some PH test strips and check before each use. if it goes above 3 PH just add a little more StarSan till it goes below 3 PH again.
oh yes 5 hours is well within its active window. you can store StarSan for several days in a sealed container and still use it. but of course check the PH first.