Invest in a good cracker, spend the 40 bucks and buy a good one. The hand crackers can be a bit dangerous when on drugs, they get very cold and can freeze your skin.
If you just want to do a couple then a hand cracker might work, but if you plan on doing many, then buy a good cracker.
It is possible not to finish off all the N2O, but not very likely. They are very addicting while you are doing them, but if they are just sitting around your house the urge just isn't there. I've kept boxes in my apt for weeks at a time without inhaling, but when the night after partying comes, boom, they are gone.
example to what Phreex said: 4 of my friends, all BL's went through 39 boxes in one night (and morning), that is 39x24= 936 bulbs, that might tell you something,

It could be a record, who knows.
another side note, I have yet to meet a person who has tried nitrous and not enjoyed the experience.