My suspicion would be in March or so, maybe as early as January or as late as WWDC in June...
I'm basing this on only a few datapoints when redesigns of the two models went off-sync. The original MacBook Pro came out in January of 2006, and a 17" variant followed in April. The Unibody 15" was October of 2008, and a 17" variant followed in January of 2009. The 15" Retina was early June of 2012, with the 1 3" following in late October. The Touch Bar models were released together in October 2016. This suggests they're often a few months apart - Apple prefers March to February for introductions. Thunderbolt got added to the whole MacBook Pro line in February 2011, then there was a minor (100 mHz) speed bump to the MBPs in February 2013 - and we haven't seen a February Mac since - while some Mac or another seems to get refreshed every March... January seems a little too soon, given that the 16" was a November intro.
If I had to guess the schedule:
March 2020: 13" Scissor MBP (the more I think about it, the more I think it may not actually be 14")
June 2020 shortly before WWDC: Minor speed bump to the 15" Scissor MBP (this was supposed to be the redesign, but Apple got tired of dealing with butterfly complaints and moved it up - Comet Lake is an easy drop-in).
June 2020 maybe at WWDC: Redesigned iMac and/or iMac Pro - not sure if these will be together or separate.
What's in the 13-14" Pro?
Near certainty: Scissor keyboard
Near certainty: New CPUs (may be Ice Lake, may be Comet Lake)
Probable: Storage bump from 128 GB on the lower model and 256 on the upper to 256/512 - maybe a 4 TB CTO gets added(???)
Probable: 32 GB RAM optional on at least some models.
Possible: 14" screen. The 13" is more size and weight sensitive than the 15", and it doesn't have the thermal issues, so there's probably a lot of resistance to making it bigger or heavier.