Servus Collie,
always depending on how tall you want to have your plants.
Normally you can calculate a stretch of 25-75%.
But I highly doubt that the White widow will perform a big stretch.
So I would say, If you change to 12/12 now, you´ll end up with a plant around 50-60cm tall.
Of course there are strains which can perform way more stretch.
Good luck for the rest of the way!
Servus👍