Watch a mbsync Maildir for changes to sync, and sync main mailboxes every minute.
Install with npm:
npm install -g mbsync-watcher
On the command line:
$ mbsync-watcher channel ~/path/to/maildir INBOX,importantlabel,anotherone \ > "[Google Mail]/All Mail,[Google Mail]/Important"
The third and fourth options are respectively:
The mailboxes / labels / folders you want to keep as fresh as possible. Generally best to set this to your inbox and all labels / folders that also end up in the inbox. These are updated every minute or on change.
A blacklist of labels / mailboxes to not listen for filesystem changes. These will get updated every 5 minutes instead.