Life expectancy really depends on how the hardware gets used. I run a small fleet of servers that are heavily CPU / GPU loaded, and the most frequent hardware failure I see is power supplies, with RAM as #2 and spinning HDDs as #3. Each of these (on enterprise-grade gear) has some amount of reporting / monitoring, but it's rarely the case that I get any advance warning before a failure. The RAM is probably the nicest, as it sometimes starts with correctable ECC warnings. Nearly all of the PSU failures have been sudden deaths, with the only saving grace being N+1 redundant supplies usually keeping the victim server alive.