I adjust my commute for a shorter version in winter. It mixes things up and gives me a chance to recuperate. Plus, I detest riding the streets in a dark evening rush hour. Call me a wimp or whatever.
All in all I keep riding but the ride and the focus changes. Plus, when it changes to 10 miles each way I have no need to pace myself and go all out for the commute.
The date that I switch is when daylight savings time ends.
I do ride my full commute when the weather is fine and I get an itch to do it.