This is my approach, stored in /etc/cron.hourly. It's very new, so I'm still testing it.
The goal is to learn HAM massages only if they are a day old, so that I can manually remove SPAM that slipped through. Mails tagged from Spamassassin are sorted automatically into the "Junk.Spam" mailbox and learned after 12 hours. I manually move any missed SPAM into the .Junk mailbox, so I'm sure there are no wrongly tagged messages there, so I can learn those messages as soon as they are found by cron.
/etc/cron.hourly/sa-learn:
#!/bin/sh
umask 022
# Learn HAM messages which were roughly received between 24 and 25 hours
ago
find
/var/vmail/fkware.de/frank.kintrup/Maildir/
-path '/var/vmail/fkware.de/frank.kintrup/Maildir/.Junk*' -prune -o
-path '/var/vmail/fkware.de/frank.kintrup/Maildir/.Sent' -prune -o
-path '/var/vmail/fkware.de/frank.kintrup/Maildir/.Trash' -prune -o
-path '/var/vmail/fkware.de/frank.kintrup/Maildir/.Draft' -prune -o
-iname '*server.fkware.de*' -type f -mmin +1435 -mmin -1505
-execdir sa-learn --username=vmail --no-sync --ham {} \;
/dev/nul 2>/dev/nul
# Learn and delete SPAM messages which were manually moved to the Junk
folder
find
/var/vmail/fkware.de/frank.kintrup/Maildir/.Junk/
-iname '*server.fkware.de*' -type f
-execdir sa-learn --username=vmail --no-sync --spam {} \;
-execdir rm {} \; \
/dev/nul 2>/dev/nul
# Learn and delete SPAM messages which were received more than 12 hours ago
# and automatically put into the Junk.Spam folder
find
/var/vmail/fkware.de/frank.kintrup/Maildir/.Junk.Spam/
-iname '*server.fkware.de*' -type f -mmin +720
-execdir sa-learn --username=vmail --no-sync --spam {} \;
-execdir rm {} \; \
/dev/nul 2>/dev/nul
sa-learn --username=vmail --sync >/dev/nul
exit 0