I usually always stay 10-15 minutes regardless of the season.. BUT you should always listen to your body!

Personally, my routine changes based on my goals and how I'm feeling that day. I believe that most of the studies on the effects of sauna and heat exposure focus on temperature and time and don't look at external environment (although I would love to read such a study if anyone knows of one!), so it's difficult to say whether there's a preferred protocol for winter vs summer. I always end sessions with what will be most helpful for my post session goals, eg. if it's in the evening and closer to bedtime I'll end with an extra few minutes of heat/sauna to help with sleep, but if it's midday and I have to work outside in the cold after then I'll end with mild cold exposure to help increase internal heat production. Do what feels good for you :)