You should be fine for 3 days. Five days in you might start getting headaches, cramp, or palpitations.
I say do a 72 then eat a meal (keto obviously) with potassium rich foods like avocado then you can do 48s and your potassium should arrive.
I normally start getting headaches and nausea at 3 days. Alternate day fasting is no problem for me without them, but not having the salts after 3 starts setting off migraines and enough pain that I wouldn’t go longer without them. Test it for yourself. Water fast until you get symptoms then break your fast and note how long it was.
If you are not doing rolling fasts, you are okay for 2-4 days depending on your metaboislm and how much you have stored up. I advise against rolling fasts without some sort of sodium, potassium and magnesium supplementation.