A quantitative (how far) answer is probably not available.
First the signal goes from the wifi router to the wifi extender and then from the wifi extender to the receiving wifi device.
Maximizing that distance depends on many things beyond just the feet (or meters) between all three.
Walls, furniture, appliances, wiring, other wireless networks with interfere with or attenuate the signal. The amount of degradation depends on the frequency being used along with the materials that the signal is passing through.
The quality of the devices can make a difference.
Most likely you will just need to experiment by increasing the distances, changing locations, etc. until the signal degrades below what you can tolerate. Gaming tends to be impacted more as failed packets must be retransmitted and then lag occurs. Not good for fast play.