Buying PC parts piecemeal is generally not a good idea for this very reason: should anything go wrong with any one part, most of your parts that couldn't be independently tested are now out of their return window. CPU-MoBo-RAM should be bought at about the same time to ensure they all work together or can be returned if they don't. One year is an extremely long time between buying a CPU and actually being able to (attempt to) put it to use.
As for how long components can sit before being "damaged by time", I have an old PC (200Mhz Pentium MMX) I hadn't turned on in ~10 years and wanted to test something on a few years ago. Its HDD's bearings had seized, gave it light sideways taps while it was trying to spin up, started turning after 3-4 attempts, was back to normal operation after a few minutes of warming up and had no problem booting into Windows 98. In general, I'd expect most PC components' shelf life to be around 20 years mainly due to flash memory leakage in subsystems' micro-controllers and configuration flash memory. Once the firmware gets corrupted by leakage, there is no recovering from it. Before that, it may be possible to reset the clock by updating firmware to refresh firmware memory.