So I summoned Durnehviir and he was flying around and decided he wasnt leaving when he was supposed too. He followed me around Skyrim for a long time, like 20 minutes real life time, and then I heard another dragon roar besides him. I looked up and couldnt see either of them. Then the new dragon fell in front of me, quite dead, and Durnehviir was nowhere to be seen. I have never summoned him to fight a dragon before, is this normal?

His timer probably ran out while he was soaring.

He gets the drain life shout at its max rank with no cooldown, so yes, this is normal.

I'm not sure about the fighting dragon part because I have never summoned him around enemy dragons but he does often stay a long time in my world too. Roughly thirty at longest in mine.

He will usually only stay a long time if you called him and there was no fighting to do.