Just out of curiosity how long can the same plane be used on a route? I mean how many continuous days usage can a plane get before it has to 'stand-down' for maintenance? It's just that I notice African Queen (G- VELD) has flown VS39/40 LHR/ORD/LHR for the past 5 days.
To my knowledge all plane maintenance (and I'm no expert) is based on hours running. Certainly my (very brief) foray into private aviation, I've done a few hours of a PPL - indicated from those sessions that the air frame needed maintenance every X hours of flight.
On top of that, the engines themselves have a set hours to overhaul. On a SEP engine for PPL - it can be as little as 1800 hours.
My understanding all of this is set by airframe and engine manufacturers. And obviously a concern like Virgin would try and keep Airframe and Engine maintenance to happen at same time.
They also have to pass annual tests - and other tests - but the hours may also expire prior to this. It could be VS's current fleet juggling is to arrange things to save on M costs on their airframes and engines. Components on aircraft are hugely expensive, so by maximising use in hours before the annual and combining them would make operational sense.
As Chicago as hours in air goes is lower than say Hong Kong, you'd expect less maintenance.
This is purely based on my basic knowledge as a previous PPL trainee - and extending this to jet's which do work under roughly same CAA guidelines to best of my knowledge.