There seems to be a lot of talk about the Colonials realizing they actually migrated from Earth to kobol. Thats a good theory and all, but it just seems like were trying to fufill an Earth is the center of the universe idea. It would be a twist, but i dont believe it will happen.
Anyway back to the main question. Its hard to say what year the earth would be in i doubt it would be the distant past like the 13 century or anything like that, but you have to factor in the dark age that occured on Earth and in the 12 colonies and then there was the ban on technological advancements with computer sciences in the 12 Colonies, but then again 12 worlds working together would allow them to advance more quickly opposed to one world that itself is divided into countries.
So, I would say Earth is either going to be on about equal scientific terms with the 12 colonies or less advanced. Not so less advanced as to be banging rocks together, but maybe just starting to expand within our own solar system. with ships of our own (not equal to that of battlestars) but can atleast make it in and out of our atomoshpere with ease.