To help you understand how you can build up to a long fast, I created a sample 3 month outline.
Week 1: Start with a 6 hour eating window for 5 days (Fasting 18 hours per day)
Week 2: Complete one 24 hour fast (I recommend fasting from dinner to dinner, or lunch to lunch, so you could start your fast after dinner on Wednesday and eat again at dinner on Thursday. Or start your fast after lunch on Friday and eat again at lunch on Saturday)
Week 3: One 48 hour fast
Week 4: 3 Day Fast
Week 5: 6 Hour eating window for 5 days
Week 6: One 24 hour fast
Week 7: One 48 hour fast
Week 8: 5 day fast
Week 9: 6 hour eating window for 5 days
Week 10: One 24 hour fast
Week 11: One 48 hour fast
Week 12: 7 day fast
Feel free to try this in it’s entirety, to shorten it, or extend it! Pray and ask God how long he is calling you to fast.