Are cows the only mammals that always lactate?

There isn't a set time for how long any mammal can lactate. Lactation is based off of supply and demand. The tissues that produce milk are stimulated by demand, like when a cow is milked, thus ensuring adequate supply at the next milking. Milk goes away once the body realizes there is no demand. This happens at weaning, and the mother "dries up."

They only do so if they are constantly stimulated to keep lactating, i.e., are milked regularly once or twice a day. If a mammal's milk is needed regularly, then that female mammal will keep lactating. If the milk is no longer needed, then that female mammal (which includes cows and goats) will stop producing milk. Besides, all mammals have the ability to lactate continuously like cows and goats (and that includes women) if they are constantly stimulated to produce milk....!!!

Are Cows Mammals

Cows aren't always lactating. They're being continuously impregnated so they produce milk.

They don't always lactate.They are kept pregnant and when they have their calves,the calves are taken away and bottle-raised or sold for veal and the cow's milk is used for humans. It's cruel,but that's how you get your milk.