I find I can run my fog machine for like a 3-5 hour school disco, fogging every 5 mins or so (You know those kids, they LOVE thier fog!!) and the fluid in the tank only drops like 1cm. It is a good idea to relpace the fluid if it hasnt been used for a while, Glycerin can smell like crap after a while!
EDIT: That fog in my avatar is actually from a fazer, they are quite effective at filling large halls etc! In the pic, we were pumpin it out at full output -- end of the concert hehehehe....