Marketing time, can, to an extent, be part of what determines arms length vs non arms length...(it is present in a number of definitions, though missing from a few definitions).
A landlord tenant sale is a grey area that can be AL or not...often, the seller/landlord is aware of what similar properties sell for in the open market, but by selling to tenant, they compensate by not having to pay a realtor comission, and not having it sit empty on the market 6 months while they try to sell. So it is an informed decision, and they are acting in their own best interest. The tenants, depending on who they are, may be more or less aware of similar places sold, but usually they are aware...they read the paper, go online, perhaps even look at a few properties before making an offer. Sometimes, I have seen tenants pay a bit more, because they don't have moving costs.
To determine how Arms length the sale is, look at the price, and perhaps make an adjustment for marketing time, and/or weight the comp less due to sale circumstances.