Rafael,
The temperature came down to the teens this past winter here in Texas. My first year Black Madeira from a cutting, was unprotected in the ground.
It nearly died. This spring it has four shoots from the main cutting and growing more vigorously than the first year. I'm hopping that the trunk puts on more mass in order to resist the freezing better next next winter. If your fig tree has sunlight in the green house then no worries, you could take it outside anytime you're ready. From my experience, it is the over watering in the pot that will kill a tree more than anything else. If you're not sure of how much to water, then use a self watering pot, the kind you find at Walmart. They work great.