Season1 to Season2 will take one second.
Quoting PezRadar:
"
Season 1 to Season 2 will happen fairly immediately. We expect S2 to start a week or two after 2.5 hits so we have a date that everyone is aligned on and knows about. S1 will literally end the second we transition to S2.
For example: Season 1 will go to 4:59:59pm Pacific Time and Season 2 will begin at 5:00:00pm Pacific Time.
"
Apparently they’ve streamlined and automated the ladder reset procedure that they always had issues with in the past with the original game. Plus it probably helps that the updated platform isn’t built on ancient code and is capable of taking advantage of modern hardware. So yeah, ~1 hour crunched down to 1 second. Noice.