When does /tmp get cleared

That depends on your distribution. On some system, it’s deleted only when booted, others have cronjobs running deleting items older than n hours.

• On Ubuntu 14: using tmpreaper which gets called by /etc/cron.daily, configured via /etc/default/rcS and /etc/tmpreaper.conf. (Credits to this answer).
• On Ubuntu 16: using tmpfiles.d. (Credits to this answer).
• On other Debian-like systems: on boot (the rules are defined in /etc/default/rcS).
• On RedHat-like systems: by age (RHEL6 it was /etc/cron.daily/tmpwatch ; RHEL7/RHEL8 and RedHat-like with systemd it’s configured in /usr/lib/tmpfiles.d/tmp.conf, called by systemd-tmpfiles-clean.service).
• On Gentoo /etc/conf.d/bootmisc.