Page cache maximum age
As others have said, there is no "correct value". But if you are really stuck; a fairly simple way to choose is to consider how often content on your site will change, and then divide that by 2. 

So if you expect your content to update every 24 hours: 24 / 2 == 12 hours. So set your cache to 12 hours. If you aren't sure how often content will change, set it to a slightly lower value. Is performance is very important, or if your site will be getting a lot of traffic, set a higher value. 

You may also want to look at non Drupal caching options such as Cloudflare, varnish cache &amp; memcache. But that's a whole other thing.