The 7X7 designation is for commerical jets, while the 8X8 series is already designated (i believe for military...) when Boeing moves from jets to ...? (Possibly scramjets) then they would most likey start a new series, like the 919 and so on. I read this in Popular Science a while back, when the same question was asked.
Someday, we'll look back at this, laugh nervously, and then change the subject.
I think that they shouldn't after 797 name the aircraft numbers, but just name them names like "Jumbo Jet" or "Sonic Cruiser" or "Mega Ark" (I know thats SQ 744F name, but it still sounds cool for a plane type.)
By the time they run out of 7's, it's likely that designing and building a commercial airliner will be such an expensive exercise that Boeing, as we now understand it, won't exsist.
Future post 7 series products will be truly mult-company, multi-national affairs.
I'm not predicting a tie-up with Airbus, though that's not impossible. More likely with LM, Russian, Japanese and other asian companies.
Sounds unlikely? Well if the 7 series last until 2025-2035, cast your mind back 25-35 years ago, and think how many big airliner makers there were then, compared to now.
I hope that boeing doesn't keep up with the boring names (says the girl with the 737 tacked on to her name) I hope that the come up with somehting completely different! Airbus should come up with a new "thingie" too!