I'd say that if the decision is so close that choosing 11 months of occupancy over 10 makes a difference in the decision to buy, you're probably cutting it too close.

In other words, if it doesn't work with 10 months of occupancy, it is pretty risky to assume you'll get 11.

However, estimating vacancy will depend a lot on the specific property and location. Are you looking at renters who will move out in a year or at renters who will stay for 2 or 3 years (or even longer)? What are vacancy rates for similar properties in your local area? National averages aren't going to help you very much.

In an average economy, using 5% to 10% vacancy (or 1/2 to 1 month) might be OK. In a bad economy, planning on 2 months of vacancy might be very optimistic.

--Peter