Anything up to 3 days is safe from start. People build up to accustom with hunger and to learn to resist it, because doing one day without breaking is better than doing 1 day and 6 hours and then failing on your goal. It’s all mental.
Just do them less often because the longer you fast, the more you lose of electrolytes with water you lose due to fast (first 24 hours usually). Ideally you want your first meal light, and not full on salt, because liver will hold onto it when breaking fast and you can hold too much water with way. And, well, with 24+ hours it’s better to take electrolytes with water. They won’t break it, but will negate the slimest of chances of problems of breaking fasts.
Definitely safe to do 24 hours fast, I do this every week on Monday. If you aren’t sure, increase gradually: maybe first 20 hours and see how you feel. Anything between 1-3 days should be safe (unless you are on medicine or have some conditions). Good lucki!!