I see two types of hill training assuming it’ll be outside. To be faster overall you generally need to be faster up the hills, and not to fade as the duration goes up. Uphill speed has a bigger impact on average speed than downhill or on the flat speed. You’ll often see riders shoot up the first few hills on a group ride but by the twentieth hill they’ve faded and are at the back.
There’s the longer hilly ride where you are working on your fatigue resistance. Thus can you climb the last hill as well as you managed the first one and not fade over the duration?
Then there’s the shorter hilly ride where you push a bigger gear (than you are used to) at a low cadence uphill to develop your strength endurance. The gear should never be so big that your knees or achilles gets strained. I usually aim for an hour of uphill in big gear / low cadence per session.
The ratio of these type of rides doesn’t have to be fixed. Vary it as suits, but above all do them consistently through week and gradually progress their difficulty.
I’m not a coach but have found these type of rides have helped me get better / faster at hills over longer durations