Mathematically, what's the best time to sell?

[Naji Boutros]

Mathematically speaking is there a best time to sell a rental property?

I figure there are diminishing returns the longer you hold a property, so it should be best to sell and buy a new rental.

However, the only variables I can think of that diminish over time is the value of money (inflation), and the tax advantages (every year the mortgage interest deduction shrinks). 

Timing the market to sell at the high point is tough, but I may get lucky.

Arguably one could say paying the mortgage off asap, and raking in a larger portion of the rent as pocket profit is nice too. 

Thoughs?