How long after first sighting of J-20 did it take to see a flight without the nose pitot tube?
Unlike the J-20 back then which went through both "2001" and "2011" stages - The J-36's first flight is done in "36011", meaning that the aircraft is very likely already an (EMD) prototype instead of just a technology demonstrator.
Assuming that the J-36 is going through the same process as the "2011" J-20:
2011 (with pitot tube) - First flight 1 March 2014 <<---
2012 (with pitot tube) - First flight 26 July 2014
2013 (without pitot tube) - First flight 29 November 2014 <<---
2015 (without pitot tube) - First flight 19 December 2014
2016 (without pitot tube) - First flight 18 September 2015
2017 (without pitot tube) - First flight 24 November 2015
So I suppose about 9 months.
Additional mention (of my own):
The first LRIP J-20 (2101) rolled off the assembly line sometime in late-2015. Assuming that point of time is late-December 2015, that's about 1 year and 10 months.
Assuming some de-buffs are applied (e.g. further technological improvement integrations and tests have to be conducted + anticipated complexity and difficulty associated with engineering 6th-gen systems) which lengthen said period to 3-3.5 years, that would mean the first LRIP J-36 unit should be ready for test-&-evaluation squadrons sometime between late-2027 and mid-2028.
This does line up with pretty well with the Adorable Whale's guess.