My experience in the business world is that you won't usually do a full (forklift) refresh until the equipment has been fully depreciated and there's some "real" need for newer equipment. This usually doesn't happen in less then 3 years and usually happens, if it happens, before 10 years. (Ten years might seem a long time, but those who have to pay the bills like "if it ain't broke, don't replace it".) Things like the desktop move from 10 Mbps to 100 Mbps (or now gig?), or moving from hubs to switches, is where you might justify a full refresh.
Along the way to a full refresh, if ever, you often do replace or upgrade some equipment. This is not uncommon and seems to fall within a 4 to 5 year timespan, as noted by the prior poster.
Lastly, sometimes businesses expand and shrink. When they expand, you often obtain current equipment. Sometimes the newer expansion equipment might also require early upgrades to other existing production equipment. If you shrink, sometimes you can re-juggle equipment, replacing older with newer. These situations, though, aren't on a planned lifecycle but they can impact your existing equipment's lifecycle.
