Landlords- Maint. Expense/Reserve?

In the 50% of expenses not including P&I payments what amount do you set aside for Maintenance Expense/Reserve?

Its difficult to put a dollar amount or % to it because operating expenses vary from proerty to property and year to year. It would be a great idea to have some reserves set aside in the beginning. Then you can add to this amt. as you collect the rents. Just put 50% of the gross rents aside and use it as you need it. Having a credit line that you use only for this property in case of emergencies could help also.