In normal specified use, the permanent magnets in a PM motor retain 100% of their field.
There are several ways to reduce the field of the magnets, all involve abusing the motor beyond its specifications.
The first is to deliver excess current. The current in the armature generates a field in a direction that opposes the field of the permanent magnets. Magnets are made from a magnetically 'hard' material, that resists this effect. The specifications of the motor are written such that when the motor is taking its maximum current, at stall, from the maximum terminal voltage, the magnets are hard enough to completely resist the demagnetising effect. A large expensive motor will often have an explicit 'max current before demagnetisation' specification. A smaller cheaper motor will usually just specify a maximum voltage, below which you should be safe even at stall.
If you power a motor from a higher voltage in the hope of getting MOAR POWA, then you might just ruin the magnets. The smart motor overdriver (I've run 24v motors from 36v in a robot competition) actively limits the current to avoid this.
A second way is to raise the motor to too high a temperature. Above their Curie temperature, all magnets lose their magnetisation. Most have a CT well above the point at which insulation will start smoking, so you're unlikely to ruin a motor this way without ruining it another way first.
Another way is to mechanically shock the motor. However, the amount of shock needed to reduce the magnet field strength is likely to mechanically damage the motor anyway.