Typically:
Between six and eight feet. That, coupled with the length of two wands, gives a goodly amount of "reach" to get at things that are up high or down low.
If you want a canister, I can wholeheartedly recommend vintage Electrolux metal-bodied ones, which are readily and cheaply available used. Parts are mostly easy to come by, too. If you want a truly dyn-o-mite canister, I highly, highly recommend the TriStar EX-20, or the earlier CXL and DXL models. These have a die-cast magnesium-alloy housing, and quadruple filtration - a paper bag, which rests inside a cloth bag, plus a motor pre-filter and an exhaust-mounted afterfilter. Wonderful machines, but they're not cheap, even used. DO NOT get the current TriStar MG series machines, which are far more cheaply made.
One other tip: Whatever machine you get, you may want to buy a couple of extra wands. Vacuum manufacturers often cut costs by supplying one set of wands that are supposed to be good for both the power nozzle and regular tools. Since power nozzles require a cord to be clipped to the wands, changing from the power nozzle to, say, the dusting brush to get at spider webs at the ceiling can be a bit of a chore. Having a separate set of wands for the regular tools eliminates that. As an added bonus, if you have an awkwardly high area to vacuum (like getting spider webs off the ceiling of a stairwell), you then have four wands to give you an incredible reach.