There are no hard-and-fast rules about the life expectancy of network equipment because it varies by device and situation. But wouldn't it be nice to know industry norms the next time you had to choose between polishing up a box or taking an ax to it?

Given the dearth of solid data on the subject, we surveyed the product-test experts in the Network World Lab Alliance and members of the Network World Advisory Board - executives in top IT spots in local Massachusetts organizations - for tips on when enough is enough.

The findings, as one respondent put it, confirm shop-honed intuition. The average life expectancy of hardware is three to five years, while equipment in fast-evolving markets, such as security, is being replaced in three years or less, and larger iron often hangs around for decades.

"We have some stuff that is eight or nine years old that just sits there and runs," says Richard Glasberg, director of enterprise communications for the commonwealth of Massachusetts. "But the rule of thumb for network infrastructure is three to five years. If you're trying to keep things humming along at 99.999% uptime, you'll question anything much older than that."

Besides upgrading equipment to stave off failure, another classic driver of change is the old software/hardware upgrade cycle, most famously witnessed in the PC/Windows realm.

"Advances in software mandates hardware changes, which serves as the platform until it no longer meets the needs of the software, so you change the hardware underneath and the dance continues," says Tom Henderson, principal researcher for ExtremeLabs in Indianapolis, a Lab Alliance partner.

Survey respondents expect Windows-based PCs to last 3.5 years on average, although some question whether that is about to change.

"There was a lot of thrash when we went from Windows 95 to 98 to 2000 and then to XP, with Microsoft upping the ante on how much resources you needed," says Lab Alliance member Joel Snyder, a senior partner at OpusOne, a consulting firm in Tucson, Ariz. "But things have been quiet since XP came out. XP-based systems may actually end up lasting five or more years - first, because there isn't a new operating system [in the immediate future], and second, anything that is XP-compatible probably has sufficient CPU and memory to run for quite a while."

Fast-evolving performance and capacity demands are, of course, at the root of many decisions to replace gear, from switches to servers and storage.

"It's not because your gear isn't up to spec or doesn't work anymore; it just doesn't do what you need it do anymore," Snyder says. "You might go buy a SonicWall firewall appliance for a T-1 at a remote office, and then two weeks later have your cable provider offer you 7M bit/sec."