Considering this question and reflecting on my own experiences, I find it takes a longer time to regain what I may lose, compared to the time I have had off (eg 2 weeks off takes 3 to 4 weeks to regain) - but it’s also dependant on what aspect of cycling fitness you’re focusing on.
By way of example, over the last few years there have been a few occasions where I’ve been forced to take 2, 3 or 4 weeks off the bike due to illness (Covid etc).
I’ve seen my FTP (as assessed by my performance on TR workouts and AI) drop between 5% and 10%.
What’s interesting is that I seem to lose more of my short power more quickly but it also comes back relatively quickly (maybe after 4 or 5 weeks).
If I take longer than that off the bike (for example I was forced to take 4 to 5 months off a few years back) then things take a much bigger hit. Not just FTP -and short power my FTP went from around 315 to 225 but my endurance and durability just disappeared.
I did some reading that suggested blood volume and other things like mitochondrial efficiency falls away when that sort of time frame of inactivity occurs and these things take a bit longer to come back.
For sure it took me a LONG time - circa 7 or 8 months to begin to get back closer to where I’d been - even longer to regain full fitness and then look to push on beyond that.
My age may have an impact (I’m 52) so rebuilding may be harder than if I was 32 - but for sure there seems to be a window in which I can stop cycling and see some preservation of ability (see summary of my personal experience below):
1 day to 1 week - no change
1 week to 2 weeks - minimal insurance loss (5%?) and some short power loss (5% to 10%?)
2 weeks to 4 weeks - 10% to 15% endurance loss - 20%+ short power loss
4 weeks plus - increasing reduction in endurance and short power with a perceived cliff edge at around 7 weeks.
When translating this to the OP question of what happens when you stop structured training but still ride, I think a similar effect applies - but with much reduced impact.
Indoor training in ERG mode is distinctly different to hard outdoor rides - so there is some circumstance specific adaptation loss, but riding hard is riding hard and I think maintaining endurance is probably easier than maintaining specific higher zone ability.
I’m fining the work around is to (in the summer when my outdoor riding takes precedent and it’s too hot to continue daily indoor intervals) that I push the outdoor volume at lower intensity and keep at least one VO2 max or maybe Threhsold / sweetspot session in my weekly plan - to keep my body attuned to the ERG training sensations.