Why does it take me forever to cook lentils when most recipes list a shorter cooking time?

Add salt only after they're soft. If you add salt too soon, they'll never soften. Using filtered water lowers the minerality of the water, probably helps in the same way.

My lentils are taking forever to good, my green lentils. You found that filtered water helped with cooking time? Thank you

also use a pressure cooker...all indian housewives or their cooks use them to speed it up!

Thanks, guys! Great suggestions - I tried using filtered water this time and it made all the difference...what a relief to finally have lentils that actually cook in the typical amount of time :-)

wWhile mine do always cook in 20-40 minutes, you might like to take a look at the answers on a previous similar question:
http://www.food52.com/foodpickle/3660-can-anyone-cook-green-lentils-in-30-minutes

I definitely feel your pain. I love beans and lentils. One time, my grandpa gave me about 10 pounds of pinto beans, and when I cooked them, the same thing happened as your story. It turned out they were his food-storage beans, and were over 20 years old! I gave up on those and tossed the rest.

I try to get the freshest beans I can, and not more than I would use in a couple of years (I have that food storage gene, so I have to at least have some for emergencies!). I've also read so many things in so many places about what not to put in the beans that I just don't put anything but filtered water in them now. I add all seasonings at the end. I know the bit about tomatoes and lemons leading to toughness, but I've also read that salt does the same thing. Mark Bittman, in How to Cook Everything, refutes as myth the theories that salt and acids with make the beans tougher, though he does say that you should wait to salt them until the middle, so they don't turn gritty and fall apart. He also says that baking soda "helps to break down the skin of beans, and acid helps keep them intact." He then adds "if you like well-defined, individual beans, include a teaspoonful of vinegar or lemon juice in the water."

Again, there are many opinions. But if lentils are taking more than 3 hours, when they shouldn't take more than 40 minutes, I dunno. I would start with buying some new lentils from a different source, and see what happens.

In Easy Beans, by Trish Ross, she says beans could be tough for one of three reasons, age, hard water, and altitude, the higher you are, the longer they take to soften.

Beans cook faster in Alkaline water. Add a touch of baking soda. Cook Illustrated mentioned this for cooking beans and course ground polenta.
Do you know if they're French Lentils those can take longer but hold their shape very well.
I use the baking soda trick all the time with dried beans that have aged a bit and just aren't getting soft....just a pinch of tho, it'll mush up if you add too much and kill all the flavor.

Beans, peas and pulses cook faster and soften quicker when you don't add salt to the cooking liquid till the end......something that took me most of my teenage years to learn :-).

Well, I'm a lover of lentils and I never cook them in tap water. I just use a big jug of Crystal Geyser or whatever filtered water is hanging around. Tap water where I live is super, super hard. But old, dry lentils will take forever to cook.

If the recipe calls for an acid, such as tomatoes or lemon juice, if you add it too soon it can slow the softening of the bean. I believe that sugar may do the same thing. Try adding it after the beans have softened a bit.

I've read that the age of beans can dramatically affect the cooking time -- generally the older your beans, the longer they'll take to cook. I'm not sure that your water would be significantly affecting the cooking time. You might want to purchase some "fresh" dry beans, like the ones from Rancho Gordo, to see how the results differ. Their beans are really delicious.