In features, we traditionally use a 2-pop at the head (with a head-format countdown) and a tail-pop 2 seconds after last frame of action. This helps confirm with the sound mixers that their tracks are precisely in sync with the show.
Some people use a "film reel" setup so that the entire film is built in 20-minute segments, and that way you're not rippling changes across thousands of shots over a 2- or 3-hour movie. Of course, at some point, when the entire film is locked, then all the reels are stacked together as one continuous show for final delivery.
Anybody who wants 2-pop audio tone can download it for free at this link:
https://spaces.hightail.com/space/DeNSB8ptCLI would recommend the 1-second 1kHz tone, and just use 1 frame of it on the 2-pop in the countdown. If you need head format countdowns, here's a link to a bunch of free ones in different formats:
https://spaces.hightail.com/space/aSyCRFor short projects, I rarely use more than a slate and 5 seconds of countdown, but it's up to the user. For final delivery, we often have no 2-pop at all and no slate, but it depends on the client and distributor.