Overnight is ideal, but even an hour or two will help.
If you don't have much time, you can speed the processs by using hot water.
Tap water, and not salted - salt toughens the skin of beans if it used before they are cooked.
Most folks discard the soaking water, as it is thought to produce flatulence.
Also, if you have a pressure cooker, you can cook dried beans without a pre-soak and get great results.
cooking.stackexchange.com/users/1393/Michael Natkin
I found this article in the LA Times with detailed experiments suggesting that soaking beans is unnecessary and it takes away the flavor and texture.
cooking.stackexchange.com/users/12572/Elias Zamaria
This website has an excellent chart showing the soaking and cooking times for various types of dried beans.
Some beans don't need to be soaked at all, while others need at least 8 hours.
I personally always discard the soaking water.
I don't like the flavors, and in some cases, colors that the water adds to the food, not to mention the dirt/rocks that settle to the bottom.
I normally soak and cook beans in filtered water, but that is only because the area I live in has lots of chlorine in the water
and I don't like the taste.
Throwing out the water also removes some of the starch so your beans don't foam so much when you cook them.
cooking.stackexchange.com/users/25418/meskarune
It depends on the beans: I soak garbanzo, kidney beans for 6-8 hours all others (pinto, navy, white, etc.
, I cook after 2-3 hours of soaking.
Here are the answers to your other questions: Should the water be just tap water, or salted?
Yes, just tap water will do.
Let the beans be immersed in the water and you should be able to put your fingers till your second knuckle.
No need for salt.
Should the soaking water be discarded or used later?
Discard the soaking water.
this will make the beans less flatulent.
cooking.stackexchange.com/users/55330/Welness Cook
For most humans, it behooves the digestive process to soak beans at least overnight.
Cooking beans w/o overnight soaking is asking for digestive malaise in most people.
cooking.stackexchange.com/users/54277/joseph rigan
An alternative to pre-soaking is pre-cooking.
I have 200g borlotti beans and tomato sauce in the oven right now in a covered casserole.
After three hours cooking at 110C
I'll take them out and leave them covered until I get home from work after which time they should be nice and soft.
cooking.stackexchange.com/users/1549/Chris Steinbach
I always soak beans because I don't like having to steam up the kitchen or leave them cooking for a long time.
Another Answer Here has a great table for soaking times.
Salt or no salt really only has to do with your personal preference, but I never do it because you can always add salt after, but you can never take it away.
cooking.stackexchange.com/users/35334/That One Actor
I read an article once that said about 16 hours is the ideal amount of time--so 12 hours plus four more.
(so if you washed and soaked them at 7 pm on Monday, ideally you would drain and re-wash and cook them at 11am on Tuesday).
This is not always possible though, given everyone's busy schedules.
I usually aim for 8 hours or more, and try to keep it less than 24 hours.
Old beans may cook easier if you add a pinch of baking soda to the soaking water (rinse it away before you cook them) because it may help soften the skins and help the water get in there to soften them.
Instead of using all the cooked beans right away I freeze some to add to baked goods like muffins (mashed before they go in the muffins).
cooking.stackexchange.com/users/75353/w. ally
I'm guessing there are different answers depending on the type of dried bean (pinto, kidney, navy, white, etc.) and I am interested in all of them.  So:  how long should dry beans be soaked?  Should the water be just tap water, or salted?  Should the soaking water be discarded or used later?  Thanks!
How long should dry beans be soaked before cooking?
cooking.stackexchange.com
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cooking.stackexchange.com/questions/6553
cooking.stackexchange.com/users/2299/Jay
While detailed experiments repored in an LA Times article support the claim that soaking is unnecessary and negatively affects the flavour and texture, it is usually thought to be important to avoid digestive malaise in most people. Beans should be soaked overnight and ideally for up to 16 hours, but even an hour or two will help. Use unsalted tap water to avoid toughening the skin of the beans and discard the water afterwards. If using a pressure cooker, beans can be cooked without soaking first. You should soak beans for at least 8 hours in unsalted tap water. To cook in a pressure cooker, you don't need to soak.
You should soak beans for at least 8 hours. Use tap water without salt. Discard the soaking water as it contains excess starch and is harder to digest. Some studies show that soaking is unnecessary and removes some of the flavour. You can cook them in a pressure cooking without soaking.