I'm another "Just when I feel like it." That's once a week to once a month, depending on how busy I am.
Interestingly enough, the more time I spend at web sites like this, the less time I spend crossdressing. Go figure!
Also, I live alone, so I could dress up whenever I'm home. But I'm paranoid about people dropping in unexpectedly, so I'm reluctant to do that. For example, several years ago, a friend called to ask if he could stay at my place for a few days, because a tree had just flattened his living room. Oh yes, he was calling from his cell phone, as he was turning onto my street. At 3am. The result was a very fast inspection to make sure all of "Mary Sue's" stuff was hidden away. That happens once, and you don't forget it!