If I'm right in thinking that the Dell 720 cartridges have the printhead incorporated in them like the HP, then they are unlikely to suffer from being unused for a time. It's printers like Epsons with the printhead separate from the cartridge that suffer. Don't remove the cartridge from the printer, the printer parks the head with a seal over the nozzles. Keeping the printer as cool as possibe will help though.
I think that you will find the Dell printer is actually a re-badged Lexmark.The cartridges have a Dell modification of an additional lug, so preventing Lexmark cartridges fitting (can be re-modified).
I leave my Lexmark for considerable times, and they always seem to work fine, when used again.
If they prove troublesome, you could try these methods. (1)Remove cartridge, place in strong polythene bag, then give a couple of arm length swings.(2)Remove cartridge, place in strong polythene bag, place in saucer/container of small amount of warm water, remove after a couple minute or so, shake cartridge slightly.
If you attempt the above, watch out for any leaks, and be extremely careful.