well, barring unforeseen events and tank busters....they actually last quite a while. i had one go about a month to 6 weeks (cleaning it about once a week or more) before i finally bailed on it. truth is, i could have gotten more out of it, but i had a several more still in the wrapper and didnt need too.
every few days, wash it in warm water and let it dry completely. or if you really want to clean it out; give it a good soak in vodka or pure grain alcohol for a few hours to really break up the gunk and that extends the life considerably.
oh..and..umm...just imagine i said something really funny here. imma hungry, so brain no work.