Yeah, 5 months. I spent the first 2 months dutifully plugging and unplugging the charger (charger out, disconnect keyboard, wait for a few seconds, reconnect keyboard, connect charger) as often as I thought of it. I was heading on an overseas flight and wanted to have the Power Cover operational for it. Obviously it wasn't working by the time I flew out but luckily the seat had a power outlet so I could remain plugged in.
Otherwise I just have the Surface plugged in when at my desk and on battery when I am commuting to work or at a meeting. Then I noticed yesterday the battery for the Power Cover showed a number other than 0% or 100%. It made my day to see it finally doing what it was built to do!
I have it charging now and will be interested to see how many hours use I can now get.