That is totally up to you =). I just have heard/read that you have to be on a water/plain tea/coffee only fast for atleast 48 hours to enter into autophagy. Since you are relatively new to Keto and your body is still adjusting to that, you might want to wait. If you think you could do that already for the 48 hours or more, then try!
You can always try, and if it too overwhelming for you and you need to stop and eat, then do that. That is what I like about fasting, it is voluntary, no one is forcing you to do it. I plan on doing a 72 hour one that I have just started at 6:00 (an hour ago) because I need the benefits of autophagy for my body. I have done 4-24 hour ones and 3-48 hour ones though so far…building up to it. I made sure I ate a good, filling meal 5:30, and had some fat bombs for desert. I will be drinking atleast 128 ounces of water daily, taking my salt, and some plain green tea and herbal tea (plain also) during that time.