Hello, in the 1900s people pronounced the year in a 2 number format ie. 1915 was pronounced as nineteen fifteen. Then, the year 2000 came upon us and was pronounced as two thousand, right now it's 2006 and we still say two thousand six, and not twenty o six. There is a famous song where the format of the year is twenty five twenty five (2525). My question is this, when do people stop using the two thousand and format, and switch to the 2 double digit format? will it be in 2100 ?

Thank you, Boris.
It happens when it happens. The usage follows what people actually say. Now (2006), most people are saying two thousand and six, but some are saying twenty oh six. There is no rule.
OK, I understand, thank you.