How do you calculate repairs/maintenance?

[Collin Wong]

When doing my analysis of a property, rental income is easily verifiable as well as property taxes, the cost of insurance, etc. However, I'm not sure what figure to use for repairs/maintenance as it can vary depending on what breaks down. I'm looking in the Denver area and a lot of the buildings are fairly old (50+ years). Even though they may have a solid rental history, budgeting for repairs makes me nervous.