From: Punta Gorda FL
Using this advice VERY loosely...
Port size 1,2,3 = 1,000 load for ships +/- variables.
Port size 4,5,6 = 2,000 load for ships +/- variables.
Port size 7,8,9 = 4000 load for ships +/- variables.
Assuming you have 5 AK's (3000 capacity) in a TF and load at a size 5 port you can average 400 supplies loaded for each ship per turn. It would take 7 days to fully load the TF of course plus/minus the factors of naval support, disruption at port etc.
There are experts on here that could break it down by ship type and so on, and I would trust thier information MUCH MUCH more than mine, but I have found it a pretty good fast n loose guesstimate.