As long as you can bear it. Which for me has been 24 +/- 4 hours between meals for 9 months and counting.
This gives zero evidence a threshold has been reached for enzyme activation.
There is evidence of effects after 72 hours of fasting [1]. But we also know of counter-effects around 24 hours, when glucose released by the liver breaking down amino acids from muscle wasting (EDIT: autophagy) partially reactivates mTOR [2]. It could be massive or minimal benefits through 24 hours, we don't know, benefits which reduce between 24 hours and some other number, and then increase until, or until right before or well through 72 hours.
[1] https://journals.plos.org/plosone/article?id=10.1371/journal...
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3390257/?_escap...
Fair enough, given it's voluntary [1]. It's biologically identical to wasting, however.
The original point was autophagy reactivates mTOR. This isn’t a linear system. It could be that a 20-hour fast downregulates mTOR more than a 30-hour fast. (We know it works at 72 hours.)
Did your overall calorie intake remain the same?