If I'm on a public Wi-Fi, I like being on as short a time period as possible, even with the "no incoming traffic" checked on the firewall. Once I'm done, I disconnect.
For Windows updates, I've found it is faster to run "WSUS Offline", which does its business extremely quickly, and you can copy the updates to a USB flash drive and update other Windows boxes without needing to burn up one's bandwidth allotment.
I think there is a utility available for the Mac where you can throttle programs like mail.app so they only use a certain amount of bandwidth at most. I think Little Snitch might offer that functionality.