What makes for a long day or a short day? We all know that good pre-production and a sensible shoot schedule is the best way to avoid excessively long days. Once in a while things can go so well that you finish early but the fact is we're nearly always cramming three shoot days into two.
But, no matter how much pre-production you do or how well planned the schedule is, the days can be really long if you're working with clients that are opposing your every move. You may have done your homework, but the client may have been so busy they couldn’t follow pre-production and are not up-to-speed on the first shoot day. If the synergy isn't there between client and crew you can expect it to be a very exhausting job.
On the other hand, it can be gratifying if you're working with a client who realizes they are part of a team. You could have long days yet still feel invigorated at the end of the day because there is an energy created by all the clients focusing on a common goal.
Don't think that doing solid pre-production will keep you on the straight and narrow. Unless every single member on the team shares the same goal, there can be potholes along the way.