How long do these last and how many are there i have already done maybe 7 or 8 i think and have 3 artifacts.

I've done like 30ish

they never end.

everlasting but you will only get a set amount of artifacts then you get junk loot

I think every 5 you get a new artificact. It goes all the way to Dawnguard Rune Axe. You have to do all the crossbow quests first though.

Okay so then im done doing them, got the axe on the last one