After Indomitus launched gw had a week of just Titanicus, a week of just black library, starter boxes, then a week of Necromunda and a week of Underworlds. They could easily have condensed some of those if they wanted to get the codexes out earlier.
The ETB kits are deliberately designed to be stopgaps until the main release and I would be very surprised if we see the battletomes before the starters and the new scenery.
I'd love to be wrong but I expect we'll see some new models in a launch day preview, but I think it will be a minimum of 4 weeks after launch until we see the new battletomes go on pre-order.