When to sell vs when to hold as a rental?

[Julieanne Zenz]

What parameters do you use when deciding if you should continue to hold a property as a rental in your portfolio vs if you should sell it?  Here is my scenario and I would appreciate any insight/suggestions:

I have a property that I've owned for 2 years.  All in cost is $170k.  Current rental rate is $2000/m, less $4,500 property taxes, this yields a net of $19,500/year (I do not have any other holding costs) and a return of 11.4% return on investment.  If I were to sell it there is a potential for earning a net profit of $90k.  This means it would take 4.6 years of rental income (assuming no major expenses) to yield the equivalent profit of the sale.  My question is what is the magic number to determine when one should continue to hold an asset vs when one should sell?  

In the past I have used 3 years as my determining factor (meaning, if the rental income in 3 years would be equal to or greater than the profit of the sale I should continue to hold the property) but have no hard data to justify this parameter?  If anyone can explain to me what the optimum ratio of rental income to sale profit is I would appreciate the insight.  Thank you in advance.  Julieanne