Interminably is how something happens when it is interminable: it goes on in a very long, boring way. “He talked interminably about his holiday” means the talk would not stop, and you wished it would. Same feeling as the adjective — too long, and dull — only this one tells you about the manner, not about the thing itself. It is a written, slightly heavy adverb; in speech people often say on and on, or forever. Though actually, like interminable, it is almost always a complaint, not a calm description of real endlessness.