Globally?Probably never.You also need to take in consideration that the new Nintendo Handheld will probably also eat the sales of the Vita.So by the time 3ds is discontinued, and its sales start to lower, so will the vita.Thats my guess at least
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1