It takes at least ten days to upgrade to level 18, because it requires 1,418,900 intimacy. To upgrade to level 18 in the shortest time, you need to spend time and money together to be the fastest. From level 1 to level 18, you need 1,418,900 intimacy points, but there are three upgrade thresholds. From level 1 to level 10, you need 18,900 intimacy points, and you can add up to 700 intimacy points every day. From level 10 to level 16, you need 400,000 intimacy points, and you can add up to 4,300 intimacy points every day. From level 16 to level 18, you need 1 million intimacy points, and you can add up to 18,000 intimacy points every day.