Although there’s no hard and fast rule for how long an indoor training session should last, we would generally advise that anything up to two hours is the limit.

This is because you work harder on a turbo trainer than when riding outdoors, as there are no downhill sections to recover. Instead you’re putting effort in the whole time. Plus, your riding position is more rigid when on a turbo trainer, meaning you’re working the same muscle groups for extended periods of time.

It is more important to create shorter, high-quality training sessions than longer ones. When it comes to turbo trainers, it’s a matter of quality over quantity.

The key to improving your average speed on an indoor bike is creating an effective training plan. Make sure that each session is high quality, as you want to avoid the indoor training equivalent of ‘junk miles’, where you’re training sessions are ineffective and don’t lead to improvements.
There are plenty of apps and pre-designed sessions you can use to ensure you’re maximising your training. Once training to a plan, you’ll gradually see your average speed increase.