I suggest you change the title to "Ask HN: How much..."

In terms of your question, there is not an exact % but rather a quite large window. I would consider these two things heavily:

> I worked extensively over the last 5 months and contributed significantly to major milestones

> My skillset is kind of a rare find

Those two things in my mind position you as a quasi-co-founder, and them offering you a 1% is a warning sign for me - meaning they might not value your contribution as much as they should.

Not having any other detail, I would suggest you to ask for a 6-8% equity, but I would also ask myself: why can't I be a third co-founder? What did they do that prevents this option in the first place?