It's different in each market AND its different in different locations within the same market, but this is a critical question.My brokerage in Salt Lake City was working with someone that came from Austin and this calculation was critical. 

Aloha,For such a critical preventative maintenance item, even IF you can get tenants to properly change filters on a fairly regular basis, you should still have a pro, or very competent handy person, conduct an annual service at a minimum (depending on local environmental conditions).

So, length of employment could be less critical.

Let me give you a bit of constructive criticism, even though human emotions may not see this as constructive.What BP should be doing, is create a board with the actual top users of the forums (if you all have not done so already), people that are on the frontlines of the battlefield and have them give input BEFORE any product is even started.

One thing I would say is that it’s critical for you as an operator to have a great track record, team, and specific experience in your asset class before inviting investors into the deal.

It provides a strong foundation in financial analysis and modeling, which are critical skills and a must have for real estate investing. 

Make sure your vetting process is critical.

Is it critical area?

Securing a good PM for an out of state purchase is critical.  

You also should add in your time for visiting your properties in person, which can add days of work but I believe is critically important if you want to be successful.