the standard in the USA is 12" from the center of the flange to the FINISHED wall (less baseboard trim). This is the most common offset, although manufacturers make toilets that will fit in both 10, and 14". Most toilets leave a space behind the toilet, so if you want it to actually sit against the wall, you'll need to pick the specific toilet and make it that much less (not recommended!). The default gap can range from zero to nearly an inch.