There's a lot of factors involved. Our server binaries can only run for so long while remaining stable, meaning we must run maintenance on them if only to restart them so that they can free up any held system resources and clear any bad states the server may have gotten into during play. We usually also need to push content updates, service patches, bug fixes, and so on which require restarting the server binaries after updating. This tends to put a hard upper limit on how long we can keep our servers running, usually in the weeks to months range. Typically we want to run maintenance as little as possible so players can play as much as they want.
That said, we have also developed clever ways of hiding server maintenance so that players don't notice it. For example, we can have "rolling" server maintenance, where we spin up a new server of the exact type that we're taking down, then direct all new logins to the new server so the old server is cut off from everybody but the players currently on it. We then wait a few hours for all the players on the server that's about to go down to log out. Once there's nobody left, we can safely take the server down, apply whatever updates we need, and then bring it back up again and allow traffic onto it once more. We can do this in a big cycle, with the players seeing only that they needed to log back in at some point and no actual downtime.
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