Using some relatively basic math and fitting an exponential regression to the previous 4 WR singles, we can expect a sub-2 by August 2028. Using the same calculations since the first sub-5 (Lucas Etter in 2015), it would be expected in August 2031, which doesn't seem very realistic. Using those same calculations for all previous WR singles (leaving out 1982), sub-2 should've already happened in May 2024, which is also not very realistic, but in the opposite direction. If you include the 1982 record, sub-2 will happen in October 2043, which jumps back to being unrealistic in the same way as measuring from the first sub-5.
Now, if you change all that to a linear regression and measure from Erik Akkersdijk's 7.08 in July 2008, sub-2 will happen in May 2028. Measuring from Mats Valk's 5.55 in May 2013, sub-2 will happen in May 2029. Measuring from Yiheng Wang's 3.08 in February of last year, sub-2 will happen in April 2028.
I could go through and average out the number of days from now sub-2 will be broken by measuring while including different WRs since the very first one, but that's a lot of work.
TL;DR, sub-2 could happen literally whenever, but I would wager early to mid 2028.
(I'd love if somebody who's better at math than me could share a better way to predict this)