How late is too late for a 3rd party POTUS campaign?

Looking at the current candidates and the lay of the land just makes me curious. How late can a 3rd party candidate can pull the trigger and still have a shot?

Well, never. No third party candidate ever has a shot, no matter when s/he enters.

But Perot didn’t get in until late February 1992, so that’s probably our gold standard. It would take astounding name-rec and a ton of money to begin much later than that.

I think that Bloomberg would have a serious shot if it’s a Sanders/Trump race and early March would probably be the last reasonable point for him to enter. However it doesn’t look like Sanders/Trump is going to happen so I doubt Bloomberg will jump in.

Similarly, in 1968, George Wallace announced his third party candidacy on February 8.

Lincoln was a third party guy. When did he get in?

Not really. The GOP was founded in 1854, by which time the Whigs were extinct. In 1860, it was down to the Dems and the Pubs. There were two third-party candidates – Breckenridge (Southern Democratic) and Bell (Constitutional Union) – but it was clear that they were the third-party candidates.