When referring to a "20k bike time," it typically indicates the time taken to complete a 20-kilometer (or 12.4-mile) bike race or time trial. The duration considered as a "good" time can vary depending on various factors such as the skill level of the cyclist, the type of bike, the terrain, and weather conditions.
For professional or highly competitive cyclists, completing a 20-kilometer time trial in under 30 minutes is generally considered an impressive achievement. This equates to an average speed of around 40 km/h (25 mph).
For recreational or amateur cyclists, times can vary widely depending on their level of training and experience. Aiming for an average speed of 30 km/h (18-19 mph) would be a good goal for many enthusiasts, resulting in a time of around 40 minutes for a 20-kilometer ride.
It's important to remember that these benchmarks are not fixed standards, and what constitutes a "good" time ultimately depends on individual goals, abilities, and the specific context of the event or personal challenge.