Tell them ASAP!
I am new to this too. But, I have err'd on the side of too early versus too late. For folks who I've met in the wild (versus online), I've either made it clear before the first date or during the first date that I'm poly. I'd rather they know early and can figure out if they want to bother exploring it further than to have them feel deceived in any substantive way.
In being poly, you have to accept the fact that many (if not most) won't choose our lifestyle. But, also have confidence that there are plenty of women who will be interested and accept you as poly.
That's where I am, at least, at the moment.
Male, Straight, Poly
“Instead of getting better and better at avoiding, learn to accept the present moment as if you had invited it. And work with it instead of against it. And making it your ally rather than your enemy.”