Is There A Preferred Cycle Time For Buildings?

There is 2 schools of thought on this:

~ Shortest Duration for AFK

The shorter the duration the more profitable the item ie you will get far more XP and Gold from 12, 1 hour items then you will from 1, 12 hour ite, so always order what ever items match up with your afk period.  This will ensure the best return for you.

~Blue Arrows

Always purchase whatever the blue arrow item of the day is for that shop as it gives bonus XP and Gold.  However there is a few things to keep in mind with this, blue arrows eat through your demand bar faster and if it is a longer period blue arrow item, quite often several shorter green arrow items give you better returns. 

It really is up to you as to which school of thought you prefer.