How long can I store whipped cream for?

How long you can store whipped cream for depends on how whipped it is. Lightly whipped cream will start to soften after a couple of hours, stiff whipped cream may last a day or so (in both cases, in the fridge).

When you whip cream, air bubbles are trapped amongst the droplets of fat, giving the cream its light texture. These naturally pop and 'leak' out of the cream over time.

As you guessed, this is one reason (the others being speed and reliability) that commercial kitchens use cream siphons. They force nitrous oxide through liquid cream as it is dispensed, so it's 'whipped' on demand. Cream whipped in this way tastes pretty much like regular whipped cream - nitrous oxide is used precisely because it is flavourless. Using, say, carbon dioxide as in soda would result in sour tasting cream.