@Aaron Mikottis
If I was better with numbers and spreadsheets, I might be able to come up with a better system, but here is what I do. Keep in mind, I am currently only tracking in one area that has about 40 active SFRs in the range I want to follow (<$1200/month rent).
Basically, I just bring a search in my specific area for SFRs that meet my rental criteria. I created a spreadsheet to track the property details of all these active listings. This includes when they went on the market, beds, baths, sq./ft., advertised rents and then notes on the properties. This might include notes about the property condition, management company or rental specials.
Once that is done, every Sunday I will get on Zillow and limit it to properties listed within the last 7 days. I will add these properties to the spreadsheet.
I will also search the previously listed addresses to see if they have been removed. I typically do a Google search for the property as well to confirm (on hotpads) that it is no longer listed for rent. Sometimes I will look on the property management company as well to make sure it was removed there too. While I won't know exactly when it came off the market, by searching every week I should be at least within 7 days.
I have a second spreadsheet that shows the same information, but also includes the date the property was removed and the total days on market.
I'm sure there is a better way to track this information, but I have not figured out what it is. My hopes, is that after some time, I can get a true indication of the rental values and vacancy rates of a certain area.