Many people use a 40" hose with a 70/90/110/swivel as their primary. It's long enough to streamline, long enough to donate comfortably, but less to manage than a long hose.
For me personally, I always have a 7' hose. It's just what I have for all of my primaries, and if I'm not diving with a can light because I'm in open water, I've got my PLB canister and it tucks there. I've never had it come off unintentionally whereas I know other divers that have had issues. I would also never dive in a technical configuration with anything shorter. Once you've got a couple stages, deco gas, reels, lights, bags, etc., there's enough going on that I want anyone I donate to, to be far enough away that I can still do whatever other tasks I need to take care of. Ascending on backgas to your first stop where, theoretically, your buddy should be off your shared backgas and onto their own deco gas, you want enough room to deal with all of the administrative tasks you need to take care of. These considerations generally don't exist in open water diving.
For me personally, I always have a 7' hose. It's just what I have for all of my primaries, and if I'm not diving with a can light because I'm in open water, I've got my PLB canister and it tucks there. I've never had it come off unintentionally whereas I know other divers that have had issues. I would also never dive in a technical configuration with anything shorter. Once you've got a couple stages, deco gas, reels, lights, bags, etc., there's enough going on that I want anyone I donate to, to be far enough away that I can still do whatever other tasks I need to take care of. Ascending on backgas to your first stop where, theoretically, your buddy should be off your shared backgas and onto their own deco gas, you want enough room to deal with all of the administrative tasks you need to take care of. These considerations generally don't exist in open water diving.