The answer is never.
That is, not in the current format of straight promotion and relegation between the Conference and Division Four.
The "experiment" of allowing non-league clubs into the football league without the need of election began in the 1986-1987 season, Lincoln were the unlucky club to be forced to take relegation into the depths of non-league while Scarborough went up. They didn't come back down again until 1999, the infamous day of May at Brunton Park where Jimmy Glass, Goalkeeper on loan from Swindon Town, scored the winner against Plymouth Argyle which doomed Scarborough to non-league football. The goal was scored so late that the final whistle had already gone at the McAlpine where the Scarborough fans were celebrating thinking they had survived.
Since then the teams that have gone up and stayed include the likes of Lincoln City, who eventually got back up, Darlington, Carlisle United, Accrington Stanley etc.... One or two clubs have been yoyo clubs, Barnet etc.... Some have gone up and eventually come back down, Rushden and Diamonds, Kidderminster Harriers, Boston United, meanwhile some went up and then, well..... Maidstone!
Some clubs have even won this league but considered themselves too good for the league and decided to stay, eh, Stevenage Borough?