There are no fixed values for this, for any antennas, as there are a wide range of factors which will affect the range;
Here are just a few:
* What connection rate is required?
* What is the client device? (it's own radios and antennas make a big difference)
* What country regulations will the AP be operating under? (this affects the power at which it can transmit)
* What frequency band is being used?
* How free from obstructions is the environment? (read up on Fresnel Zones)
* At what height is the antenna deployed, relative to the clients?
* What existing usage of the WiFi bands will the new deployment need to compete with, for airtime?
The recommendation is always to plan well by getting answers to such questions, in advance - then perform a Site Survey (i.e. test your preferred solution, in the environment in question, using the devices to be used after go-live). Remember too - connection rate does not equal application throughput.
As has already been said; for 10-15 metres, with direct line of sight, ANT-20 will be fine