During “normal” times, looking at data over a few months is totally reasonable. With the current global supply chain issues, data over a few weeks is hardly even viable. Your only good option is to look at what things are doing right now. If the deal wasn’t done in October, it’s useless.
Also, with the turn over going on in the industry currently, the only reason a dealer would have a car on a lot for a year is because they are absolutely insane with their asking prices. It used to be that a long time on the lot was a good indicator of a potential deal. Now, it’s only a gigantic red flag of a dealer with unreasonable expectations.