Check out the Ultimate Tech Tree in the CivRev General Discussion forum. The first GP is at 150 culture, the second at 266 and the third at 400.
Note: GPs only count towards the cultural victory when they are settled in a city, not if they are able to move. Using a GP doesn't count towards a cultural victory, although their works may (i.e. flipping a city or building a wonder).
If you get Great Persons through tech (first to monarchy, invention...), you actually get the amount of cultural points currently needed to create a great person.
Say you need 266 cultural points for the next great person, and you are at 166 cultural points when researching monarchy.
When you finish researching monarchy you get 266 cultural points. The first 100 CP completes the great person.
And the remaining 166 points becomes your boost towards the next great person.
Since 400 cultural points is the next limiit to reach, you will have 166 of the 400 CP.
I have the I-Phone version and I don't think there is a way to tell where you are at with respect to culture. The version is adequate if you aren't home or near your xbox, but is lacking in quite a few areas.
The worst things in my mind for that version are;
1.) Not being able to sell units.
2.) Not being able to settle next to barb huts.
3.) No status screen access as described above. (Right trigger menu on xbox.)