After a few days without electrolytes, your body won’t be able to send electrical signals efficiently, by about a week or two, your heart will arrhythmia and you will die without immediate medical attention. In my opinion, fasting beyond 3 days is not worth the risk to me, but I am not in any way telling anyone else what to do, only the facts that you will not do well without electrolytes.
Since I’m doubting that you are going to do your own research at this point:
Electrolytes are essential for basic life functioning, such as maintaining electrical neutrality in cells, generating and conducting action potentials in the nerves and muscles.
Add the electrolytes to your water and sip it throughout the day during any prolonged fast.