Before pfctl acquired the ability to expire table entries, Henrik Gustafsson had written expiretable, which removes table entries which have not been accessed for a specified period of time.
One useful example is to use the expiretable program as a way of removing outdated <bruteforce> table entries.
You could for example let expiretable remove <bruteforce> table entries older than 24 hours by adding an entry containing the following to your /etc/rc.local file:
/usr/local/sbin/expiretable -v -d -t 24h bruteforce
expiretable was quickly added to the ports tree on FreeBSD and OpenBSD.
If expiretable is not available via your package system, you can download it from Henrik's site at http://expiretable.fnord.se/
as security/expiretable and sysutils/expiretable, respectively. It is also worth noting that expiretable was removed from the OpenBSD ports tree in 2008.