Here is a vast simplification: Neutron star is the ashes of a star, it can't burn any further. It has energy in the form of rotational kinetic energy and it has energy in magnetic fields. All of those will radiate away over eons. Eventually it will be a cold, stable mass.
Neutron Stars have a compaction of 10^17
It has been observed to show properties of a Quark core.
When a core of a star collects enough matter, to form a Neutron Core, the size will develop Dipolar Electromagnetic fields strong enough to expel in the form of an hourglass its solar envelope.
The spin of a Neutron Star, is understood by understanding Chiral Supersymmetry that exhibits Dipolar Electromagnetic Vector fields, expelling matter away, and attracts matter to it.
Neutron Star is not the ashes from a star.
How long will it burn?
Billions of years.
If it's allowed to burn, in time the dipolar magnetic fields lose their power allowing the expelled matter to collect and a solar envelope will form.
If matter is collected than the Neutron matter will break down to quarks, compact to a Quark star and in so doing increase its dipolar vector fields and the vector fields pulling in matter forming a mimic black hole where EMR cannot escape.