When you
cuddle a
cat for an
extended period of
time, and he is
purring loudly, you can stop cuddling him and he will continue to purr for a
short amount of
time afterwards. This is known as
purr inertia.
The amount of time the cat continues to purr is directly proportional to the amount of time you were cuddling him, T, and his contentness level, C, by the following equation:
(T * C) * 10 = P
P is in seconds. The amount of time is also dependant on how comfortable the cat is. YMMV.