In general, the longer the better. A longer duration increases the likelihood of sessions to be served from the cache. For example, say you’ve set cache duration to 1 hour, and you get 10 visitors an hour. In that case, during a 2 hour duration, 2 visitors won’t get cached content, and 18 will. As a result, these 2 visitors will experience slower page load-time. OTOH if you set the cache duration to 2 hours then only 1 will get non-cached response, and 19 will be served by cache.
The downside of a longer cache duration is that content can become mare stale. For example, if the page uses Wix Code on the server-side to get the weather and display it in the page, a cache duration of 1 hour will cause weather to be up to 1 hour out-of-date. If cache duration is 2 hours, then it can be 2 hours out-of-date.