Boyles law is applicable when
a) Temperature is constant
b) Gas is at high temperature & low pressure
c) The vessel enclosing the gas is a good conductor
d) The process is isothermal

Boyle's law states, PV = constant. It is applicable for ideal gases, which means temperature is high and pressure is low.

For isothermal process, temperature is kept constant and heat is slowly added/removed to/from the system which has to be conducting.