It took a while to get that lead from Glenumbra for me the first time (not as long as 120+, though), and the second time, it dropped on the second ore I harvested.
It's going to vary. Widely.
But that can be said for every lead in the game.
Heck, RNG is like that in every game that employs it, not just ESO.
As someone who's grown to detest grinding, I decided on something for this type of lead. Rather than grinding for hours trying to get a lead to drop from mobs/nodes/chests/etc, just do them for a few minutes at a time, in between other more enjoyable activities. I can't count the number of times I've failed to get a lead to drop after 30 minutes to 2 hours of grinding, only for it to drop within the first 2 or 3 tries the next day.
RNG in this game is painfully stingy. I've never understood why either. It's not like they are giving anything away with higher drop rates. They'd just be making the game more rewarding and more fun with better RNG.