How soon is too soon:

Never!

I usually start packing things into my suitcase at about 3 weeks out.
However it's likely that stuff has been getting piled into a staging area way before that.

I packed 90% at 27 days, the only bits left were the things I needed to use or wear in between. If it stands stil pack it

we usually go the end of september so my cases come out about June!!! and I just pack, unpack, pack, unpack until its time to go!!!

but usually the "final pack" happens the weekend before we are due to go but you can always "practise pack"

I already have some stuff packed for next years trip

I think you are late! Chop chop!!

my husband is the same way!

I must admit that most of our packing is done the night before as well!

Dh does his case about 6 hours before the flight. I make my list about three weeks out but don't put clothing in the cases until the night before. I really prefer my clothes to be as fresh as possible. Med bags are checked at the three week mark so I can update all the little things like band aids during normal shopping trips.

We are like Jan, we pack the night before.

I'm a night before kind of girl. I usually need the stuff that I'm going to pack so packing 3 weeks out would be impossible for me!