The power of the heater will determine how fast the plastic will melt and get to temp. The equation is
Q is the heat given off
m is the mass of the plastic
c is the specific heat constant
Tf is the final temperature
Ti is the starting temperature
The specific heat constant can be found here. It changes based on temp but you use the average of you initial and final temperature.
c= 2.073 J/gK
Plug that into our equation we need 352410 Joules to melt the plastic.
A watt is a J/s
Say our heater is rated to 175W, it will take 33.56 minutes to heat the pot. This does not take into consideration losses through the walls.
Concept 2, the pot has a capacity of 2-3 kg of plastic. So the 175W heater would take 2-3 times longer to melt.