Here is a thread on that subject:
Indoor trainer sessions - how long? Training With Power
I’m sure this has been discussed, but I can’t find it searching… When the plan calls for a 2+ hours training session and the weather won’t allow it outdoors, are people riding that long on indoor trainers? Or are y’all modifying the session for that day? I’m not sure I can handle trainers sessions longer than 90+ minutes. Thanks!
The main point is probably cut down for 3 hours to 2 hours max. I would most likely recommend a Zwift group ride. I’ve been doing quite a bit of those this year for some solid zone 2 riding. It’s a bit more controlled and most likely you are not looking to get in so much anaerobic work at the moment. If it is a bit more zone 2/3 that is ok. Avoid going close to or over your FTP.
When you get closer into season you can do some races, or substitute a lot of sweet spot to hit TSS. Kind of depends when in the season you are and when your first race is. If not racing till April or May no reason to crush yourself indoors at the moment.