European Mandrake Germinated: Now What?

After about two months, one of the European mandrake (Mandragora officinarum) seeds I planted has sprouted. How long should I leave it in the refrigerator before bringing the seed flat out into the light and room temperature?

Probably the best course to get the most seedlings woud be to prepare another pot with a humus rich well-draining soil of pH about 7.5 and scoop the seedlings that have germinated out of the flat in the fridge and plant them in the pot (handle them only by their leaves, never the stems), water well and put into a bright but not sunny window.
If you are satisfied with one or two seedlings, the safest course would be to take the whole flat to the bright spot as soon as the first seedling germinated. Some more may germinate a bit later even when out of the fridge, but it can't be guaranteed.

Will it survive outside in zone 6?

It is hardy in zones 6 to 8 and with a mulch should survive in your area.