For a little while, depending on what you are seeing. A crank that is a touch slower than usual, on a battery that still starts the car every time, can wait a few days for an appointment. Plan on sooner rather than later, though, because a weak battery tends to give out on a scorching afternoon or the first frosty morning. A swollen case, a rotten-egg smell, a battery light while driving or a car that needs a jump to start are different: those mean stop putting it off and get it tested now.
Putting it off seldom makes the bill smaller. A car that dies in a parking lot can mean a tow, a missed shift and a rushed decision. Each deep drain also takes life out of the battery, and repeated jump starts make the alternator work overtime to refill a battery that cannot hold a charge, which wears it too. A leaking or swollen battery can corrode the tray and cable ends around it, adding parts to the job. Catch it early and it is one battery. Get ahead of it at the closest C&C shop, where a quick test settles the question.