The online Collins dictionary shows that it is in the 10,000 most common words. So it is in the vocabulary of a typical middle school student.

It is a fairly common word, where I am anyway. Why would you like to know this? :)

Yep. A regular word, used/heard fairly often.

It is common, but it depends on which meaning. It can be a noun meaning a kind of food: "I put relish on my sausage." (a condiment like ketchup and mustard) It can be a noun indicating delight or pleasure: "I greeted my cousins with relish." It can be a verb also indicating delight or pleasure: "I relish meeting new people." The first one (the food) is probably the most common in conversational spoken English. The second two are not that common in conversational spoken English, at least in North America, though they would certainly be understood. I doubt that I use "relish" when talking with my friends.

I can think of two meanings. Relish can be a fruit-based (mainly tomato-based) sauce. That makes it a common word. But it can also mean something like enthusiasm and appreciation. 'He bit into the apple with relish.' In this context I think it's a literary-register word. That is, you probably won't hear it used this way unless someone is reading aloud from a book. (not common)