Chris,
15,000 miles. I had my 7 DVA`d in August. I did one airfield day where I had a problem with an air lock, as water shot out from the expansion bottle. I had it serviced the next day, and was not used for three months as I had to change the CRB. When I started it up I had white traces on the dip stick. I had noticed that the expansion bottle was empty after the lay up. I can only assume that I have a minor failure in the head gasket, as I can`t believe that condensation would make such a mess of my oil. I now have a bleed valve fitted. Simple moral to the story, make sure you bleed ALL the air out of the system.