Work out journal size instantly with clear inputs, formula shown and shareable results.
systemd-journald caps the journal at the smaller of SystemMaxUse and 10 percent of the filesystem by default, and rotates the oldest entries out when the cap is reached. Retention is therefore the cap divided by the daily write rate, not whatever MaxRetentionSec says — the size limit always wins.
Journal retention
needed = daily volume x retention days; allowed = filesystem size x SystemMaxUse share; achievable days = allowed / daily volume
Because SystemMaxUse or the 10 percent default caps it by size. MaxRetentionSec only sets an upper bound on age, it does not reserve space.
Yes for anything you need to investigate after a reboot. The volatile default in /run loses everything on restart, which is exactly when you want the logs.