I've heard that white men can't jump. I've heard that lightning never strikes twice in the same place. I've heard a lot of things, some true and some ridiculous. I've had pullets continue laying through their first winter and older hens that finish the molt early and start laying lay quite well on 10 hours of natural light. You may be able to guess how I feel about that 14-hours-of-light myth.
What causes production to drop in the fall is not the length of the day but that the days are getting shorter. Nature's normal cycle is that the hens lay well and raise chicks in the good weather months when the days are getting longer or are pretty long, then when the days getting shorter signal that the good weather days are soon over they stop laying eggs and use the nutrition that was going to make eggs to replace their worn-out feathers. Then they wait until the days getting longer in the spring signal the good food times are back. We've messed that cycle up some by domesticating them and breeding them to lay eggs instead of going broody a lot but practically all hens follow that general pattern of laying and then molting.
To keep your hens laying your goal is not to give them a certain amount of light but to stop them from seeing the days getting shorter. It is late September and I assume you are in the northern hemisphere. It sounds like some of your hens may already be molting. Those will probably complete the molt before they start laying again. But if you increase the hours of light now you may be able to stop others from starting the molt. I don't think you need to increase the current length of days, just stop them from getting shorter.
Somed people extend the hours mornings only, some evenings only. Some have a set time in the morning and a set time in the evening. Your days are going to continue getting shorter until late December, you may want to consider that.
Hens can only lay for a certain amount of time before they have to stop and recharge their bodies. That's why commercial operations have to decide whether to replace the flock or force a molt to maintain profitable laying. Their margins for profit are pretty thin to start with so they carefully track laying to determine when that point is reached. The signs they are looking for are reduced laying and they start getting more weird eggs. I don't know how long you can go without a molt, each hen is different. But at some point you will start to see issues so be prepared.
Good luck!