Very rarely. The only place I notice a difference is if the last letter I type is a j, the cursor 'hesitates' for a moment because Vim is watching for a k.

The advantage I found of jk over jj is that, in normal mode, jk is a no-op, so if I hit jk as Escape when I was already in normal mode, it doesn't matter.

The only problem is that when I'm editing text out of Vim, I end up with the occasional jk at the end of my typing. That almost happened while writing this comment.