My i1D3 is about five years old and I've taken good care of it (when not used I've kept it in a closet in it's original package) but I wonder what's the average lifespan of i1D3? When should I start to look for a replacement? I don't own a spectrometer,

As the i1D3 uses glass filter, the lifespan is basically until you drop it, and break it, or the USB cable becomes fractured/snaps.
In many cases, the older probes are better than the new versions, as they have better low-light capability.

Steve