From what I have read, if you shake it up and it foams up easily then it is still good. I always refill my spray bottle each brew session and then use it for spraying down my beer thief, new airlocks, etc. Next brew day, when I make a fresh batch I empty and refill the spray bottle. Since I brew every two to three weeks, the stuff doesn't get a chance to get old. I also make a spray bottle of cleanser that I use to spray empty bottles before putting away.
"So you say you just brewed your first batch of beer. Welcome to the obsession." --me, to every first time brewer I ever meet.