It realy depends on what type of system you try using them on
say system 6-7 need more than one becouse they are inneficient, a 4fd on a sys 11 will give you 6-9 months backup
8fd on a sys 7 boardset will give you 2-3 months
a 1.5 on a bally -35 will give you 6 months, or over a year with a 4fd charge time varies leave the machine on say 2 hours youll be pretty much loaded