I may have let Arch go months without updates before. Its not advisable to apply them on a schedule either.
Providing regular prompts to apply updates seems like the best solution.
This cronjob will download packages and cache locally. Put into root's crontab or run via sudo.
0 2 * * * /usr/bin/pacman -Syuwq
Drop this line into .bashrc
and you will be prompted to apply updates once per day.
# Prompt to update arch linux once each day
[ -f ~/.update/$(date +%F) ] || (mkdir -p ~/.update; rm -f ~/.update/*; touch ~/.update/$(date +%F); sudo pacman -Syuq)