In my opinion, as often as possible. High-end routers like those made by Cisco quite simply just work, making life for you and your customer that much easier and stress-free. For most customers, the features of consumer-grade routers, such as those from Linksys, are usually sufficient, but having a router that just works really is what maters, and that is where Cisco routers deliver.
So here's the thing about consumer-grade routers: they do work -- most of the time. I have never had a router that simply did not work at all. These routers will chug along, but then one day something weird will happen. I've seen it all: from DHCP failures, to NAT issues to VPN tunnel nightmares. Although powercycling usually does the trick, you have an unhappy customer calling you here and there wondering why there are constantly problems, and you end up spending a lot of time trying to figure out what is wrong, only to find it is something you generally cannot fix.
To be fair, I have had many installations that use these entry-level routers problem-free. However, the few that have had problems are enough to warrant playing it safe and going with something that just works. And with basic Cisco IOS-based routers starting at $200, cost really isn't that much of a factor.
My only words of caution: Cisco routers are not as easy to set up as the consumer-grade routers, and setting them up incorrectly could lead to big security and functionality issues. Nonetheless, once you get a Cisco router up and running, you and your customer can pretty much forget about it.
This was first published in November 2007