Speaking from the perspective of a woman who has been on dating sights, I have to say the right time to tell someone you are poly is on your initial profile.
First, it is honest. But second, you are not looking for a non-poly relationship anymore than the monos who see your profile are looking for a non-mono relationship.
It saves everybody time.
Also, I have to say that if I saw "I am poly and seeking a second lover" on a profile, I would at least feel curious enough to check it out. On the other hand, if I was told about it after I go through the hopes, the anxeities, and the hassle of meeting the person, I would be, at a minimum, annoyed. And probably for that reason alone lose interest.
I also think that poly guys really have to be careful about how they come across when telling woman that they are poly because I have met a lot of guys who, when they say they are poly, come across as arrogant SOBs who think they deserve more than one woman. (this is rarely the reality, but it is often the first impression that is given.)
So, telling the viewer of a profile right off the bat what they are and are not getting into by talking to you is probably your very best bet.
Best of luck in your search for a second!