The sweet spot is something like this https://www.redfin.com/KS/Lenexa/12211-W-79th-Ter-...where it's entirely rent ready, few upgrades so it shows pride of ownership... but it's outdated.

Based on these publicly available data and the market-rate construction cost, I think ~650 sqft is the sweet spot where you can maximize the ratio of rental income/construction cost - you can build an ADU for under $250k and rent it out for at least $2500/month.That said, if you are building an ADU for self use and prefer as large as possible then it's a different story. 

If your current property doesn't produce cash flow as a LTR after paying a PM and maintenance and cap ex reserves and vacancy, then cash in on that appreciation while you have that sweet, sweet capital gains tax exemption (terms and conditions apply, talk to a CPA about whether you actually do qualify in your specific situation). 

Get a positive cashflow property with another $20k tax free/deferred in your pocket after 6 months is a pretty sweet deal.

Things get a little different though for more expensive properties.The 'sweet spot' for OKC homes is around $100K full retail. 

I guess I'm asking if I should be looking at listings expired for 30, 60, 90 days or more.Another question I have regarding looking for leads on the MLS: If I want to target old but active listings, is there a days on market that you find to be a sweet spot?

Lower middle income neighborhoods can be a sweet spot as far as rentals go.

Today, I went on Zillow and saw a listing for a sweet double wide.

Wiring additional funds to get a sweet deal?  

But, lowballing a home that has been on the market for 90+ days and is vacant, estate/probate or distressed in some way can be a sweet spot.2) Doing an ARV for every single property you want to lowball will get annoying if you expect a report and analysis.