you can leave them for a while.... don't worry tho here's what i did when mine stretched... i don't know what medium you're gonna use after the rock wool cubes but if you use soil this will work..
i started mine in little cubes like that... when you begin to see the roots showing out of the cube it's time to put them in the larger ones... after that to your medum or you can skp the larger ones and go straight to the medium... when i put mine into the soil i covered them w/soil all the way up till about an inch or two below the first set of leaves... the rest of the stem will actually root also, that way it somewhat eleminates the stretch....