Had another little play today, by spamming the screenshot keys I kept a Solo instance just outside a ring station paused for 10 minutes before deciding that was long enough. Everything was fine but I was unable to request docking of enter SuperCruise (hmmm, didn't try Hyperspace, must try that next) so the Solo instance must've become de-synced from the BGS.
After more trials it seems that it's linked to the maximum time a Solo instance can be disconnected from the internet (which appears to be around 2min 7sec maximum). I've carried out successful pauses of 2 minutes in SuperCruise, 4 km from a ringstation with 10 NPCs visible and in a variety of Signal Sources.
Even in an empty instance the game still 'calls home' about every 2 minutes, why is that do you think? Can't be updating the BGS as there's nothing to update...
There's actually loads of mini changes in the BGS all the time, but I dont see the need for updates that frequently, other than for commodity pricing I guess.