If you’re burning, let’s say, 2k calories a day – 15 calories is 0.0075% of 2k. There’s 1440 minutes in 24 hours, so 1440x.0075. You’ll have calorically powered 10.8 minutes in a 24 hour period with your multivitamin. Fasting/autophagy/ketosis aren’t a lightswitch that’s on/off.