How long does it take for MacPorts to catch up with new macOSes?
Unclear exactly. You could try searching their GitHub repo for exactly when support was added, but because of the way it's laid out, good luck.

Here's the best info I could get using The Wayback Machine as to when the release first crops up (not necessarily the actual release date):


10.12: 3 days after release
10.11: 9 days after release
10.10: 4 days after release
10.9: 20 days after release
10.8: 1 day after release
10.7: 4 days after release


Prior to this the capture points are too scattered to be useful. Based on how short these numbers are, I suspect that development was done on beta versions. Hopefully it's coming soon.