Just throwing this out to the community to see what others feel is a reasonable amount of time for a server to be online. We have a number of mini servers that we've been running for three years then rotating them out for new. We max out processor, RAM, and have been using SSDs for years now with an external drive for most backups. They work like a charm, but we've been rotating at three years in part because they are on 24/7 and that's when the warranty runs out. We have kept a few as development/test servers past this timeframe, but we've had those for upwards of five years without a lick of trouble.
So, how long do most people keep servers before putting them out to pasture?