When transplanting from one pot to a larger pot you should first see roots hitting the bottom of the pot. When the first root/taproot hits the bottom off the small pot it will signal to the plant to start producing more roots, these will then grow downwards and outwards at the same time and will soon hit the bottom of the pot also.
So when you see say, 10 roots at the drainage holes, you will know that your plant has established a good root system in that pot and it can be transplanted into a bigger pot.
Someone has already said to flush with distilled water to leach the excess nutes from the MG soil. I would make sure the water is between 6.3 and 6.8 ph also as that soil is probably either too alkaline or acidic.
And finally if it was mine I wouldn't risk keeping it in that soil. I would remove the soil from the pot, wash it away gently under the tap, and very gently place your seedling and root into new soil. You have to be very careful doing this tho, and quick. So unless your confident you won't damage the root then just go with the flushing.
Good luck.