I thought this might be an instructive exercise, and having asked my database to do the arithmetic I was presented with a slightly alarming result...
It may not have helped that in the last month I have sold two and bought one.
Here's my result -
Report for time owned
Number of entries: 8
Minimum value: 0.08 years
Maximum value: 1.52 years
Average: 0.61 years
If I remember, I might run this report and poll again in a year's time to see if I improve!