... Of course, I know it's a simple matter of figuring out how many days I would use the lens in a given year, and multiplying the result to figure out how many days it would cost for a full purchase. ...

Actually it is more a question of how much you are going to use the lens in total during the next few years.

Let's assume the worst case and put the rental price at 100USD / day. You would need a total of 100 days of usage to make it worth buying that lens.

Now, a 800mm is a lens that has a long potential usage period (high build quality, unlikely to get replaced by a better lens during the next couple of years, etc.) So let's say you are going to use it for the next 7 years (again, worst case). If you say that you are going to use it more than 100 days during the next seven years, buy it. Otherwise, rent it.

This is the simple calculation.

If you want to make it more complicated, you need to factor the following things in:

* resale value after a given number of years

* chance of theft / loss / breaking

* chance of switching camera systems

* possibility of you shooting different subjects in a few years

* etc.

* additonal costs for insurance and added equipment (new backpack, new tripod, new head, new sherpa)

you get the picture