document max. value of SigningKeyLifetime
tor's manpage says:
SigningKeyLifetime N days|weeks|months For how long should each Ed25519 signing key be valid? Tor uses a permanent master identity key that can be kept offline, and periodically generates new "signing" keys that it uses online. This option configures their lifetime. (Default: 30 days)
It does not include information about what is the biggest acceptable value. Tor simply fails to start if the given value is to big:
[warn] Interval 'XX months' is too long [warn] Failed to parse/validate config: Interval 'SigningKeyLifetime XX months' is malformed or out of bounds.
Please also mention if there is a value for SigningKeyLifetime where it is actually less safe than running in non-OfflineMasterKey mode (maybe it is less safe to set it to 10y in OfflineMasterKey mode than to run in non-OfflineMasterKey mode?) and if it makes any sense to modify this value in non-OfflineMasterKey mode (because that is apparently possible).