It's usually pretty firm at about a year. Until then the plastron has a bit of give to it, but it's not really soft.
I bring my whole plastic tub habitat out during the summer of the baby's first year - out in the morning, back indoors at night. I keep this up until he's big enough to live in the yard. That depends upon the species. A baby sulcata might be big enough at 2 while a baby Texas tortoise is still in the house in a tub at 4, going out in the a.m. and back in at night.
The shell on a baby sulcata will normally feel hard and loose that "leathery"/pliable feel in just a few months. Certainly by 5-6 months. As Yvonne says, by a year it should be very firm and hard.
I do not transition mine to full time outside until they are about 8", 3-4 lbs. That would be their second year. But a lot will depend upon where you are located, you local weather, and how you have their enclosure and night box set up.
Greetings. Being outside permanently depends primarily on three major factors. 1) age/size and 2) location as well as 3) what kind of enclosure and house you have created.
If you have a one year old sully that weighs 5 lbs and live in Michigan....right now he better be inside. If you have a three year old, 35 lb sully and live in St. Augustine Florida, he better be outside.