Can range anywhere from 1 minute to half an hour, anything below a D tier is immediately obvious after the first song :) I usually spend more time if I'm not sure where to put it, I have more time with the unit and/or I feel like it shines in certain situations and I gotta go through different songs/genres to justify where to put it.

As for equipment I almost always use my LG V60 with HibyMusic playing flacs. Personally I've found that it supplies more power than the Apple Dongle and some units sound marginally more "energetic", but personally I guess only 15% of ~B tier units benefit from this, definitely more as you go up the chain. Sometimes I don't have full direct access to headphones (especially the expensive ones) and I can't jack in 3.5mm. For those, I go through all the tracks I recognise well enough on the store supplied DAP and make my judgement there. Though I definitely skipped on a few just because I don't want to mislead anyone just because I couldn't test them properly enough.
More info down in "Info/Test Tracks" by the way!