It is a good practice to test your DR plan regularly by failing over to your DR site. I would suggest doing this failover test once a month, but this may not be possible for all enterprises, at the very least, you should be doing it once a quarter to gain confidence and peace of mind knowing that you have a solid and reliable DR plan that works. You have no idea how many clients I have been to that tell me they have a solid DR plan but when we test it they rarely work! This mind-set exists because too many companies believe in set it and forget it, as we mentioned previously, you must test your DR by failing over to your DR site to ensure that it works.
You wouldn't want to find all the issues in your DR plan in the middle of a disaster. Backups alone are useless if you don't have a place to restore them. The way to go from ought to work to known to work is through testing!
The reasons for infrequent testing are usually budgets and the scarcity of time. This is why most failures are usually discovered during a disaster. And at that time you have a few or no practical alternatives.
A well-developed Disaster Recovery plan will identify all key processes and steps to failover to your DR site. It should have a predefined schedule for testing, after each test document any weakness found and what was done to correct them.
New technologies, such as virtualization and cloud computing make regular and (even daily) testing feasible. These technologies allow you to automate processes and provide a foundation for an ongoing RTO and RPO reporting at the management level, allowing you to better estimate and mitigate risks for the business.