For Geth v1.9.0, look here for some new details on sync time. Describes all three modes and times including some changes that one should know about.
As of Aug 2, 2019 the size on disk for me was 356 GB and state entries went to 370 million. Takes a couple of days to sync with a ssd drive. Be patient.
Look at the Geth Source code,
# StartingBlock is the block number at which synchronisation started.
# CurrentBlock is the point at which synchronisation has presently reached.
So the startingBlock is the point at which you originally started syncing i.e. when you first loaded your client , while the current ...
Just to make sure: the chains could not connect if they didn't have the same genesis file.
If you now connect the nodes to each other they have to reach consensus about what is the canonical chain. So they start to go through the regular consensus protocol which basically says that the longest (actually heaviest, but doesn't make a difference here) chain is ...