I doubt anybody is filling semen tanks with nitrogen on a 16 week fill, let alone longer; the week rating is for an empty tank.  Every canister you add increases nitrogen consumption and reduces hold time.  Every straw of semen displaces nitrogen and reduces hold time.  I know SS fills ours every 6-8 weeks and there is no way the rep would risk a unit running dry by pushing the limits.  Our rep also services a lot of dairies and needs to vist them more than 3 times per year, hence the 6-8 week fill fits the route better.