That is great that you are testing so frequently. Nice to hear. It really depends on your environment and your requirements. When I was a customer and partner helping customers I usually found it to be once every 6-12 months, though this was really more to keep the business happy, from my standpoint a test of the recovery plan was sufficient.
A failover/failback might make sense after upgrading, though it wouldn't necessarily have to be the entire plan. It could just be done for a subset or test group.
As I mentioned at the beginning though, it really depends on your comfort level and your business requirements.
That sounds reasonable.