What soaking duration for clams in salt water you never exceed?

I phoned supermarket, and seafood monger said "DON'T soak clams and mussels for same time". She said "you can soak clams overnight, but not mussels". She got no idea why 3/4 mussels died. They opened and smelled nauseous.

I'm baffled at all different duration recommendations below, from 20 mins to days!!

1. What's maximum soaking duration for clams in salt water? What soaking duration will kill them?

2. What's best soaking duration?

How to Clean and Cook Fresh Clams - Chowhound

1. Fill the first bowl with cold water—never warm or hot, since that will kill the clams—and submerge all the live bivalves. Some people add cornmeal to the water to help purge the clams, but all that should be necessary is salt (if even that). You can add enough salt to make the water mimic ocean salinity, then put the bowl back in the fridge and let the clams soak for 20 minutes.

How To Purge Clams, The Nitty Gritty Of Shellfish Smarts

3. Purge for at least 1 hour. I find this time sufficient for clams bought from the fishmonger. If you leave them for significantly longer than that, check on them from time to time and change the water so they don’t die from loss of oxygen. When you tap or agitate them, they should close (perhaps slowly, but they should eventually completely close). The last thing you want is to forget about them and come back to a bowl of dead clams.

How To De-Grit and Clean Clams • Just One Cookbook

5. Pour salt water into the tray/bowl. The salt water level should be right around the clam’s mouth. If there isn’t enough salt water then make more using the same ratio of water and salt. Cover the top with aluminum foil leaving some space at the corner for air. Keep it in a cool dark place (or fridge) for 3 hours. This should give the clams enough time to purge sand and grit.

Steamers {Soft Shell Steamed Clams} | SimplyRecipes.com

If you don't have time to let the clams soak for hours, just put several of them at a time in a large bowl, cover with salt water, and gently swirl the water around with your fingers for half a minute.

How to Clean & Store Clams & Mussels | Tasting Table

Most hard-shell clams will be fairly grit-free. However, says, Peterson, "steamers and razor clams benefit enormously from an overnight soak in salt water." It has to do with their anatomy: Steamers (or soft-shell) and razor clams never close because of their siphon necks. "The sand isn't just in their shells—they actually ingest it, too," Branchina says.

Duarte Farrajota Ramos

Where I come from (Portugal) it is quite common to see in most restaurants and establishments shellfish being soaked several hours to even days long before consumption, especially sand dwelling shellfish, which is not the particular case of mussels.