Probably not that much later than Airbus' available delivery slots?
I think the problem is the efficiency sought.
- 737NG already has CFD wing
- the new engines are same for both
- fuselage won't add up to much, the tail has already been tweaked thrice.
So you can do weight.
- But a composite hull (787) for a shorthaul a/c needs to take a lot of beating on the ramp. Also the aftermarket needs aftermarket repairs, availability of both affects the sales price. Sales price needs to cover the development cost and here the two go against each other.
- go fly-by-light, all electric FCS. Sadly the recent even shows Boeing and their ecosystem do not have the skills to achieve that, present day.
- a plethora of unspecified small improvements. Which are also available to Airbus on their current airframe, hmm.
So all new airframe then, once materials and technologies that are not available now become suitable for mass production and field deployment. At the same time when 320NEO is becoming obsolete, not sooner.
my 2pc.