I've seen trucks with 15' stacks (or at least close to it -- cause they made mine look short). Just checked CFR 393.83 and the fed law is silent. Might be a State reg but I've never heard of it -- or it could be company policy.
To be sure, if you want to avoid overpass hassles, keeping your stacks under 13'6" is the best policy -- but it don't look as cool!
Anyway, never had it called during inspections -- and the boys in Washington look for everything.