It's based entirely on my workload. No sense in interfering with my paying customers if things are steady. But when things get slow, I'll take on the occasional "mastering test" mostly because I consider any time spent at the console, means I'm hopefully getting at least a little better or more experienced at mastering.
One thing I don't do, is send back an entire song. I know people have mixed opinions on that, but that's what works best for me.