Everyone is different when it comes to healing and anatomy. Just keep in mind earphones are not only kinda gross (ear waxy) but also can put heaps of pressure on the inside of the piercing.

I have had clients (with larger ear canals) that have used them without problem the entire healing time, but it does depend on what jewellery you have in there and such.

Short answer, wait till you have it downsized and its healed - so a couple months minimum.