You can cap it as many times as you want, as long as the casing is sound and not too old.  One of the companies we buy from has a customer who they cap tires for about ever 3 months on certain yard applications where they are scrubbing all day long.  They get as many as 9 treads on a casing before it's done. 

It's highly unlikely you could fly somewhere and back without landing on at least a couple tires which have been capped more then twice.