you can easily take this into account too, and my calculation did. Assuming your tenant is staying for 2-3 years, you can really only afford to let it set empty for a few months. Your yearly rent increases are just a couple percent (tenths of pecents a month). Every month it sits empty is 8% of your yearly income for the unit.
The yearly income isn't what's important to these landlords, it's the real estate value of the property. Loosing out on a full year or more of rental income is the cost of doing business if it keeps the underlying valued high and appreciating.
