Will this ruin my progress? If not how long to dry fast for?
There doesn’t seem to be a unanimous verdict on how much muscle mass you lose, if any. From the multiple sources I have looked at they agree that dry fasting retains muscle mass better than water fasting. I will link if I can find the source but I have read that 90% of your energy during a dry fast comes from fat and 10% from protein, keep in mind that muscles are not the only source of protein in the body.
From my personnel experience after doing a 5 day dry fast I did not notice any muscle loss however gym’s are closed here and I have been unable to test my strength post fast. Next time I need to cut I will definitely use dry fasting. Because even if you do lose some muscle mass you can go straight back to bulking and making gains instead of doing a 6-8 week cut where you end up losing muscle mass and missing out on 6+ weeks of making gains.
Start with 12 hour dry fasts every other day then move up to 12 hour dry fasts everyday then increase fasting length when you hit weight loss plateaus. You will lose a lot of fat the first 2-3 weeks doing just 12 hour dry fasts.