You can send letters to AMCAS as soon as it opens I believe. However, schools that use these service (at least this cycle) were few and far between. A good rule of thumb would be to have your letters ready to be sent (i.e. through Interfolio) by the time you submit your primary AMCAS - some schools will get back to you very fast (like GW) and you shouldn't have to wait to fill out their application due to a delay in your letters!