Actually, the amount of T-SQL to update will depend greatly on how close the development team followed best practices. My organization recently upgraded from SQL 2000 to SQL 2008 with literally less than 10 code changes on a database of over 5,000 objects. The database has been operating without any problems for roughly a month now.
I would recommend running the SQL Upgrade Advisor to give you some idea of the impact on your situation in regard to length of time. From my past experience, ANSI-style JOINs, if you have them, are the most common problem to fix, as they are not supported in 2008.
SQL Server 2008 Upgrade Advisor can be found here:
Good luck!