You can patch tubes until you're placing patches over the old patches, if you're patient and frugal enough ;-) However, the big seam splits and damage at the valve stem almost always mean that you finally need a new tube.

The "quick" or glueless patches (essentially the lick 'em & stick 'em kind) don't bond to the tubeand depend a lot on the pressure of the inflated tube on the tire in order to hold a seal. I don't recommend theseif you're going to patch the tube and put it aside for use as a spare.

Vulcanizing patches (the black & orange kind that use glue) actually bond to the tube, and the bond works better if you give it 12 to 24 hours for the glue to set up. These work well if you put in a new tube when you fix the flat, then patch the old tube and let it sit overnight.