The cached content for guests if configurable, so you can have it cached for 1 minute or X hours if you choose. Keep in mind that there is a global TTL (time to live) on all cache too which overrides everything, so if you set guest cache to 2 hours and global ttl to 1 hour - then it would only keep for one hour - the opposite way around however would work fine (1 hour guest, 2 hour global).
Thirdly, if your opcache operator has a configured TTL that will override all TTL settings within vB Optimise, making that the longest possible TTL.