Most maintenance breaks last for a few minutes or seconds, but there have been reports of breaks lasting several hours, days, weeks. In rare cases, months.
Reasons for maintenance breaks can range from mere bug fixes to new features being added. During the 2010 merges, the affected hotels were under maintenance break while the merge was underway. In rare, extreme cases, a website may be closed for many hours or even days during a major exploit in security. This has been reported in late 2008 when Habbo US has been under attack by a hacker.
Maintenance breaks are different from connection issues, in which the entire community cannot connect to Habbo instead of one player. Maintenance breaks also can't be fixed by basic troubleshooting such as rebooting your computer and clearing your browser cache.