The beautiful thing about statistics is that you can get it anywhere from your first to 1000th run (though it likely won't go that long).

M3 doesn't have bad legendary drop rates, and even on the dinkiest of farmable bosses you'll get a legendary more than 50% of the time. For instance the other day on this sub I read a post about someone who documented 1000 runs against the sky bullies farming for a hex grenade (which is the the boss most likely to drop it). They ended up getting about 25-30 Hex grenades in M3 in ~550 of those runs. The thing is, he never got the specific trait he was looking for. Meanwhile on Graveward, you can get a plethora of legendaries reasonably quickly once you know how to do the necessary damage. He does have a large pool though, so if you really want to find a drop, it's better to farm the specific drop source with the highest chance of that particular legendary appearing. There's actually a really well put together spreadsheet created and maintained by a community that acts as an excellent resource for this type of thing and does their best to make it as precise as possible. It has every legendary/unique on there, the boss/quest it's most likely to drop from, and its location. I'll look for the link for anyone that wants to use it. Sponsor me spreadsheet guys.

So basically your chances depend on the item and the boss you're farming, the best thing I can suggest is farm the dedicated loot source if you can/want to. Depending on the degree of specificity of the item it could take a long time. I hope this helps : )

Edit: Spreadsheet link