Absolute Minimum: 5 Inches
The OSHA limit on radiation is 5 REM / year, and surface radiation on Mars is 20-30 REM / year.
Using normal concrete, you'd need about 13 cm / 5 inches to reduce 20 REM to 5.
Realistic: 2 Feet
5 REM / year is still a lot of radiation.
50 mR / year is probably a more reasonable long-term value. So we're talking a three order of magnitude reduction - which is roughly 63cm or 25 inches of standard concrete.
You can play with "stay times" in different rooms. For example, if you make a two story habitat, then the upper story protects the lower story - all the air, the floor, and any equipment in the upper story absorb radiation that would otherwise be a problem. Similarly, exterior rooms shield the interior rooms.
So if you put the places where the colonists spend the most time (bunks, kitchens, etc) on the interior, and put places where the spend less time (atmosphere control, storage, waste-water processing, etc.) on the exterior, you can get away with thinner walls, with the understanding that people should restrict their time in the exterior areas.
Note that total does matters, so time outside the habitat would have to play into this calculation too - it could get fairly complicated trying to compute the actual answer, since you'd have to predict how much time people would spend in each area, and how material might move around the colony over time. Spare parts get used eventually, right?