Are they paying market rent, are they excessively long or about to expire, etc etc. 

@Cj PowderhornHi CJ, Regarding your Cape Coral experience, the median time on market for duplex rentals for the April to present time period was 37 days, so yes, your vacancy period for that unit was excessive.

You are trying to make an estimate so you're not caught flat-footed.Calculate your plan and increase it by 25%.

It will typically last long enough that it goes out of style before it needs replacing from excess damage.  

This works reasonably well for many situations, but it can lead to issues when you happen to have a particularly wasteful tenant that uses excessive amounts of water and/or electricity.

(they do charge a flat $2 for ACH, but tenants will pay that).

The brokerage offers an appealing Flat Fee 100% commission program.

Speaking as both an investor and realtor based in FtL, these small MF are very difficult to get into at the moment due to excessively high pricing leading to negative cashflow if you plan to finance them. 

As a Property Manager in a market where many owners choose to manage on their own, I've seen several examples of owners losing their homes or properties because their tenants did not pay or caused excess damage.

I did talk to a couple trying to buy a $1.4+ million duplex/2 level flat here and house hack it (they were outbid).