A: It entirely depends on the temperature. If you can leave the fermentation jar in a cool location (74 degree or lower) then 3 weeks is totally fine. It will undoubtedly be tart, but you can simply sweeten it back up with sugar/ fruit after in a second ferment.
As for a second fermentation, we recommend no more than 3 to 5 days. If your bottles are stored in a warm location, try to keep it closer to 3 to avoid too much carbonation.