Ok so first off how long is Durneviir supposed to last? I summoned him near Fort Dawnguard and figured he would last a few minutes, but I swear I left the fort and went off and played for at least an hour and when I came back to save at the fort he was still flying around... thought that was weird maybe a glitch that would get fixed by reloading.
But when I loaded up my save file the next day he was still there! And that scenario basically played out again(played for awhile came back to save and he's still flying around) this went on for at least 2-3 game sessions before he finally disappeared.

Now when I try to summon him he appears for maybe 3-4 seconds and then just dies before he can take off or anything(I already mastered soul tear) so basically I can't use him because he just instantly dies/returns to the soul cairn. Has anyone else had this problem? Or maybe can you no longer use his summon after you learn all of soul tear?

You can still summon him just fine after you learn all the words to Soul Tear, maybe he wore himself out flying around Fort Dawnguard with nothing to fight.