From what I understand it doesn't last that long if you mix it with tap water. I had a bucket go south over night. It made everything slimy that was soaking in it.
If you mix it with distilled water it will last for around a month or until it goes cloudy. I have a gallon of distilled water starsan mix that I always keep on hand. I pour it in a spray bottle and use that for a lot of the sanitation. It was the best bang for your buck addition to my brewing supplies period.
Best was to tell if it's still good is with ph strips. You can buy them off eBay for around a buck a pack. They are not super accurate but close enough to tell if sanitizer is still good.
bad67z posted while I was typing so I guess +1