At least 36 hours worth
http://www.nintendoworldreport.com/news/41846/super-mario-maker-under-maintenance
A bug in the level creator for Super Mario Maker has necessitated a day and a half of online downtime.
Nintendo has sent notices on their various websites advising that an unspecified bug in the "Create" mode has necessitated a stop on all online services. The maintenance is currently slated to end at 8 p.m. Eastern tomorrow night (1 a.m. GMT on Friday morning).
A mandatory software update will be available for the game upon completion of the maintenance.
So if I ever find a major bug like this I should just ignore it or actively exploit it. Nintendo clearly does not want you to make a good effort to inform them without revealing the bug to the whole world.