If you submit your primary ASAP, not all will be available at the same time, thankfully. Some you may even be able to work on before your primary is completed (Yale comes to mind, if I recall correctly). Many will be available later in the cycle as schools begin to accept and request secondary apps.
Keep in mind, you should put thought and effort into each any every essay that you write. If you are just 'going through the motions', it will be obvious. Much like the MCAT preparations, it requires some discipline to get through many essays.
However, at this point in the game, I would focus on your Personal Statement. Without a good PS, you may not even get Secondaries. No sense putting the cart before the horse. This, of course, is assuming you are an applicant for this upcoming cycle. If not, I don't think your cart is even in the same county as the horse...
Good luck.