Unlimited. Radio signals never disappear altogether; they just attenuate beyond detectability. If you had an infinitely sensitive receiver and some way to filter out the noise, you could pick up WiFi from the moon.
But you don't have an infinitely sensitive receiver nor a perfect noise filter. If you want to know what the practical range will be in your specific environment with your specific client devices at your specific signal strength and SNR requirements, you'll have to do a site survey.
With that said, Cisco's very rough rule of thumb for indoor installations is 465 square meters per AP for data traffic, or 370 square meters per AP for VoWLAN, location services, or other link-sensitive applications. Those numbers are only intended to give you a starting point for planning out your survey, not to render the survey unnecessary.