honda 400ex smoking

Well sometimes a new engine will smoke some from all the cleaners and chemicals used in the cleaning process but it should clear right up usually after the rings have seated into position( break in) run it for a little bit (2 or 3 hours) nothing heavy then change the oil. If it smokes still then you have an issue eg. ring spacing, valve seats

How bad is it smoking and how long after the rebuild?

15 min isn't long enough for break in run it longer and change the oil and filter this will remove any impurities , oil in the exhaust will take awhile to clear. let us know