How long can 5 months old kittens stay outside

I am a new cat mom, and have two precious kittens at 5 months. We have cat proofed our balcony, and let them out on it a few minutes a few times during the day. They get more and more exiting about being there and watch the traffic, birds and neighbors passing by. However; I live in a cold, northern, rainy country (temperate between -3 to +3 degrees Celsius/ 26.6 - 37.40 d. Fahrenheit). How long can I let them stay at the balcony? They got their vaccines (they got their last one Friday). They are both raised inside. I really want them to enjoy the balcony, but don’t want to risk their health because they get too cold. Right now I lure them in after a few minutes outside.