We use it as a general day to day water conditioner. with every water change, don't use anything else (May consider using Safe or Ultimate in the future, but haven't yet and haven;'t needed to)
you can use it generally, can use it for emergencies, it just stinks.
I think when we last ordered it, we got either a gallon or two. and we really do not have that sort of water volume in the house (or do that many water changes) so it lasts forever.
I'm really not much of a fan of adding chemicals to an aquarium if I can help it. prime and liquid fertilizer for the plants is really the extent. Or salt for a marine aquarium