How often should I run integrity and consistency checks on SQL Server 2008?
For me, it depends on your maintenance window and usage. It takes time and resources to run the DBCC checks so you want to minimise imnpact on a running app

If you can run it every night, do so. Otherwise weekly. 

Or monthly if you really are 24/7.. but then I'd be running DBCC on a restored database as an extra check.

And you have to consider other maintenance too: indexes and statistics in your window