In my expereience carobs can be picked upto a year(possibly longer) after they fall. It all depends on the conditions and what you want to use them for. You can easily tell if a carob is any good just by the weight and texture. Weighty and shiny is how you want them. If they are dull or black and rough and/or have no weight to them then they are no good.
If you have a tree that hasn't been harvested for awhile then dig around amongst the old and obviously useless carobs as you will be surprised at how many good ones survive underneath. For the carobs to survive any length of time they need to be in the shade and have some element of moisture under the tree. If you are planning on chopping them up for animal feed then some of the tattier carobs are fine too.
Bruixot