It depends on the brake.
Muzzle brakes can be quite effective in reducing recoil, particularly if they are designed well. However, there is no definitive answer to this question as the amount of recoil reduction will vary depending on the specific brake and the firearm it is attached to. Some muzzle brakes are more effective than others, and some firearms are more sensitive to recoil than others.
This differs depending on the type and design of the brake, and the cartridge for which the rifle is chambered. A muzzle brake is most useful and effective on calibers ranging from the centerfire 22’s (5.56mm, .222, .223, etc.) up to the sub-30 calibers (.243, 25–06, 6.5mm, etc.).
Also, remember that some designs are not very effective and you need to do a little research to find the most effective one.
That said, a well-designed muzzle brake can often decrease recoil by 50% or more. And for those who are really sensitive to recoil, that reduction in recoil can make all the difference between being able to enjoy target shooting or not.
Here are some tests that will show it clearly:
See also: the AmmoCave team picked up the best recoil buffers for you!