A few things that might help you.
It takes an egg about 25 hours, give or take, to make its way through the hen’s internal egg making factory. It can only be fertilized during the first few minutes of that journey. That means if a mating takes place on a Friday, Friday’s egg is not fertile. Saturday’s egg might or might not be fertile, depending on the timing of the mating and when the egg started its journey. Don’t count on it. Sunday’s egg will be fertile. This is after a successful mating. A rooster does not mate with every hen in the flock every day, but he doesn’t need to.
The last part of the mating is when the hen stands up, fluffs up, and shakes. That fluffy shake gets the sperm to a special container near where the egg starts its journey. That sperm can stay viable for quite a while, normally from 9 days until possibly more than three weeks. Most of us count on two weeks. Three weeks is possible but it’s a real stretch. Don’t count on more than two weeks.
Now to get to your unanswerable question, how long will it take for a specific rooster to mate with 12 specific hens? It depends on the individual rooster, the individual hens, and how they are managed. A young energetic rooster will probably be able to keep more hens fertile than an older more laidback rooster. Individual personalities make a difference too. My current one year old rooster has trouble keeping eight hens fertile. If a hen runs away when he dances, he just quits. He doesn’t chase after her even a bit so some hens never get fertilized. He is quite a disappointment. I’ve seen where one rooster kept over 20 hens fertile. Practically every egg I set developed.
A rule of thumb is that hatcheries keep one rooster for every ten full sized hens in the pen to ensure fertility. The ratio for bantams is normally somewhere around 12 to 15 hens per rooster. This is where they may have 20 roosters in a pen with 200 hens. This does not mean that one rooster keeps 10, 12, or 15 hens fertile. In the pen breeding system the mating is random. Some roosters will do a lot more than others. They constantly monitor fertility and may add a younger rooster if the fertility rate drops. The new competition stirs up the older roosters and improves mating activity. There are a lot of variables involved.
You are not in a pen breeding situation. You only have 12 hens and one rooster. As you said, your rooster was keeping the flock fertile. He only had to mate a hen once every two weeks to maintain that. Who knows how long it will take him to cycle through every hen when he comes back, if he does. Things have changed. Your hens have been without a rooster for a while. He’s been to the farm and will have returned. The flock dynamics may be different this time.
What I suggest is you check the eggs that are laid for the bull’s eye. If the eggs you open are fertile, the eggs you don’t open probably are too. When the fertility rate is where you want it to be for incubation, incubate some eggs. I don’t know what your criteria is for how many eggs you want to incubate or what fertility rate you need before you are comfortable starting incubation. This thread helps explain what you are looking for when you look for the bull’s eye.
Fertile Egg Photos
Another option may be to just get some fertile eggs from another source, maybe the place the rooster is now. That would be a lot simpler but you may be determined to hatch your eggs.