It isn't clear yet as no snapshots even exist for 1.16, as 1.16 I assume is probably fairly big but not 1.13 or 1.14 big (maybe smaller but not 1.15 small) I'd assume it might take a few months.
1.14 took from around October 24th 2018 (first snapshot) to April 23rd 2019 (1.14 Release) with it's blocks, mobs, world gen, loot tables (dungeon/temple or mob drops I mean), technical aspects (performance/bug fixes), commands/datapacks, localization (language is important but we can't forget it as players since we only focus on our own language but it is part of game design so can't leave it out as it might have taken more time then the content in the updates themselves for all we know) and however much more.
Depending on the content besides what we already know being nether biomes and mobs (with particular AI when you have gold armour), and no clue about new blocks being added to said biomes; it might not take too long but again we don't know if more is planned then what has been said or if any of those features will be part of 1.17 for all we know.
Without snapshots or more clear info we can't tell. Just use mods to get a similar experience in 1.14/1.15 I'd say but otherwise what we know is what was announced at Minecon.
1.15 took a lot longer than originally projected, and from what i could see in the big "reveal" in September, the changes will be in the Nether, but there will be new biomes, new mobs, new mechanics in the Nether. Looked pretty big to me. But hey... My crystal ball has been cracked for years.
I assume that the plan is to release the Nether Update before September.
There's a lot of content to be added, refined and debugged, and the same to be done for Bedrock and China as I assume they'll release their updates around the same time, but my uneducated guess is that if the developers resist the urge to make major technical changes also it should be possible,