Re: How long (miles/gas) should breakin take
If your bottom end is already broken in and you're just breaking in a fresh top end, I'd go to 16:1 for only a tank or so, warm the thing up gently, then ride HARD for a while. Alternate the engine speed a bit but mostly ride really hard to seat the rings, after that tank you can go to 25:1 or even a bit leaner if you wish and ride as normal.
Everyone has their own opinions as to break in but this is mine. Rings break in quickly but should be run hard to do it properly. On a new engine you're balancing running the engine hard to break in the rings quickly vs running it easy to break in the bearings and lower end gradually. Since the bottom end is already broken in, I think you can just run it hard to break in the rings without worrying about the bottom end.