I had a fractured fourth metatarsal a few years ago that i managed to do surfing, and a fractured fifth metatarsal from an impact coming down from a jump and landing on the edge of the board this season.
Fourth metatarsal was in winter and I wore my boot and rested it, did swimming for exercise, took about six weeks to heal. I felt like I was ready in week 5, but fortunately there was no wind and the extra week made a massive difference, from small amount of pain to nothing at all.
This time, it was during summer and it seemed a shame to waste a season so i just padded up the moon boot and went out anyway. Its at about six weeks now and will need another couple of weeks I reckon. It has definitely set me back but totally worth it.
Those were full fractures so I reckon 12 weeks would be a bit of a stretch for a hairline unless you've done something really interesting to it, although they are all tiny little bones and a greenstick fracture can become something much worse quite easily. The main thing i've learnt is its really easy to get overconfident as the pain stops and then to overdo it and set it back a lot.
If you are stupid enough to go out kiting, don't go for too long and try not to jump. The pain will stop after a while and you won't realise how bad it is until the next day when it becomes apparent that you've really screwed yourself.
Its probably self-evident that this is a really bad idea and you should follow your doctor's advice, but you didn't come here for a medical opinion, did you!