If you're not sure where to start, assuming you're currently cleansing, perhaps consider your eating habits for example. Have you discovered you don't actually need your morning coffee? Or were you eating breakfast out of habit, but find you can easily fast until 11am? How about that afternoon packet of chips and chocolate bar? Could you swap them for a banana and a quick walk around the block to reinvigorate you instead?
Winter is the season of slowing down, which can mean greater time for contemplation (assuming you can avoid Netflix for half an hour - maybe you could contemplate your Netflix habit while you're there lol). you can think about general topics such as "Am I happy where I live?" "Do I feel fulfilled with my work?" "Does my house feel comfy or cluttered?".
There is no magic amount of time or topics to journal about, if you're new to journaling, try 15 minutes on one topic. As there are 3 months of Winter, you could choose a new topic each week. You may find that once you delve into a particular topic it can be hard to keep up with the pace of thoughts coming into your mind! Other topics you may not have anything to write about. There's no need to force yourself to write. The only thing I would suggest is being honest with yourself about why there's nothing to write - is it a topic you honestly don't have anything to write about? Or are you avoiding looking at the topic?
You don't need to make journaling a daily habit. It may be something you only do when you're pondering a particular problem Many people do enjoy taking the time to inwardly reflect every day. Some enjoy a morning journaling ritual, while others prefer to journal before bed to reflect on their day.
Have you tried journaling? What's your favourite time of day to journal? Is it a ritual for you? Let me know! :)