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.

On CentOS (and I assume Fedora), there's a job in /etc/cron.daily called tmpwatch. This runs /usr/sbin/tmpwatch, which will delete files that haven't been accessed in the specified number of hours, i.e., the default behavior is to examine the atime for the file to evaluate if it's been used recently.

http://linux.die.net/man/8/tmpwatch

Other distros (and installations) may have /tmp mounted as tmpfs, which is an in-memory filesystem. This will get cleared on boot.

On Ubuntu 11.10 which I'm using, there's an upstart script in /etc/init/mounted-tmp.conf. The start of it says this:

# mounted-tmp - Clean /tmp directory
#
# Cleans up the /tmp directory when it does not exist as a temporary
# filesystem.

description "Clean /tmp directory"

start on (mounted MOUNTPOINT=/tmp) or (mounted MOUNTPOINT=/usr)

You can read in more details, however in general /tmp is cleaned when it's either mounted or /usr is mounted. This regularly happens on boot, so this /tmp cleaning runs on every boot.

In /etc/default/rcS you have TMPTIME set, which is used in the above init script to feed the two find commands at its end - basically controlling file deletion based on their times (modified, changed, accessed).

From Fedora 18 on, /tmp is mounted on tmpfs (i.e. RAM) by default, and thus erased on power off.

This behaviour can be disabled by issuing systemctl mask tmp.mount and reboot (and reenabled by issuing systemctl unmask tmp.mount and reboot), and then /tmp will be mounted on the / filesystem and can be controlled by /usr/lib/tmpfiles.d/tmp.conf settings.

See http://fedoraproject.org/wiki/Features/tmp-on-tmpfs and man tmpfiles.d for more details on each case.

On RHEL 6.2 the files in /tmp are deleted by tmpwatch if they have not been accessed in 10 days.

The file /etc/cron.daily/tmpwatch defines the way tmpwatch is called.

#! /bin/sh
flags=-umc
/usr/sbin/tmpwatch "$flags" -x /tmp/.X11-unix -x /tmp/.XIM-unix \
    -x /tmp/.font-unix -x /tmp/.ICE-unix -x /tmp/.Test-unix \
    -X '/tmp/hsperfdata_*' 10d /tmp

The -x arguments are files to be excluded. The 2nd to last argument is the time to wait after the last accessed time. The last argument is the directory to examine.