We have a DR drill every six months. All critical systems are tested every time. Others go through a rotation with the expectation that everything will be tested at least once every few years.
If you are short on money, consider using virtualized systems for your test. Remember, testing is not necessarily about the speed of the test system, but the ability to recover the data. In the event of a real disaster, the plan would include purchasing real hardware, but the recovery steps would be the same.
Our last test was a few days after the Irene and Lee dumped 8 inches of rain in two weeks. Our biggest concern: will the bridges be open so we can get to our test center.