Since the initial post on 1/20/2017, this thread has grown and grown. Out of curiosity I looked at the dates for various "milestone" post numbers.
5000th post 5/25/2017 (125 days after 1st)
10000th post 8/4/2017 (71 days after 5K)
15Kth post 12/15/2017 (133 days after 10K)
20Kth post 2/16/2018 (63 days after 15K)
25Kth post 4/20/2018 (63 days after 20K)
30Kth post 6/16/2018 (57 days after 25K)
35Kth post 8/17/2018 (62 days after 30K)
40Kth post 10/14/2018 (58 days after 35K)
45Kth post 1/3/2019 (81 days after 40K)
50Kth post 2/21/2019 (49 days after 45K)
55Kth post 3/30/2019 (37 days after 50K)
60Kth post 5/17/2019 (48 days after 55K)
65Kth post 8/1/2019 (76 days after 60K)
70Kth post 10/1/2019 (61 days after 65K)
75Kth post 11/4/2019 (34 days after 70K - a new PR!)
Averaging over 73.6 posts/day since 1/20/2017, or about 68 days per 5000 posts on average.
Averaging about 70.3 days/5000 posts so far