I still get a little giddy thinking about diving back into 'Mystic Messenger'—it's one of those games that feels longer than the clock suggests because of how involved the chatroom rhythm is.
If you're talking a single, proper route from start to a 'good' or 'true' ending, expect roughly 8–20 hours of reading depending on how fast you skim and how many chat windows you join. The story is spread across in-game days with scheduled chat times, so a lot of the perceived length comes from waiting for events in real time. If you follow every chat, respond to late-night invites, and hunt down all the weekend photos and special moments, a route fills out more naturally and can drift toward the higher end of that range.
Now, if by full playthrough you mean clearing every main route, secret routes, and extras (including bonus story packs or 'Another Story' content), you're looking at a much bigger time investment—think 60–120 hours depending on replays for bad endings and how much time you spend savoring the texts versus speed-skipping. Personally I paced myself over a few weeks, sinking in for nights and finishing some routes in marathon sessions on lazy Sundays; it made the chats feel like hanging out with friends rather than just ticking boxes.