^ Important to mention that each SS13 server is a self-contained codebase; everyone is basically hosting a different version of the game itself. So HRP and MRP's biggest differences lie in how the actual game is structured.
In MRP (and LRP), the game is usually centered around the gamemode "secret" - there may or may not be antagonists playing in the round, and if there are, the type of antagonist (traitor, blob, changeling, wizard, etc.) is unknown and there may be multiple. Random events are peppered throughout the round at varying severity, which usually means that the round generally HAS to end sometime between 45 and 150 minutes after it starts because the station slowly becomes more and more uninhabitable.
In HRP, the game is usually centered around the "extended" gamemode - no antagonists, random events are far less severe (but still sometimes detrimental), and the game can go on practically indefinitely, like that 8-hour long round. This gives the players a lot more room to freely roleplay and independently decide when they wanna wrap up the round, usually after most of the starting resources have been used up and the station becomes cluttered with random items on tables and floors from its previous occupants.
In any gamemode, you can jump in/out of the game at basically any time, so you don't NEED to stay for the whole round and you can jump in at the middle if you'd like. Most people play in short bursts but I'm unemployed.