We're just starting out so we don't have a good repair cost history. Some components are brand new and others might be 20 years old, so we're planning for a higher-than-average one.
We're planning on retiring in a year or two so the fire hose of monthly salary cash will be gone. For that reason we're planning on about $5k per each of our first 4 properties. After that, we'll add $2k to the kitty for each of the next 4. That should give us a good sized buffer (than can be raided to buy another house if it isn't needed).
Until the buffers are in place we're routing all rental payments to the buffer.
Your mileage may vary... :)