Yo MoggersT!
That's an exciting stage to be at! Generally, you want to wait about 2 to 3 weeks until the seedling has developed at least two or three sets of "true leaves" (the jagged ones) and a strong enough root system to handle the elements. It is super important to "harden them off" first rather than just tossing them straight into the sun; start by putting them outside in a shaded, protected spot for an hour or two a day and gradually increase the time and sun exposure over a week. Keep a close eye on the nighttime temperatures as well, making sure it stays consistently above 15°C so the little one doesn't get stunted by a cold snap. Once she looks sturdy and is used to the breeze, she'll be ready to take on the great outdoors full-time!
Happy Growing, Growmie!