In some commercial temperature controller (ex. Yokogawa ) when you said that the control is time proportional it works like this:
1.First define a period T ( as Raghu state this period will depend on the thermal constant of the system).
2.Let suppose that the output is a relay ( may be a solid state or electromechanical) that once activated gives power to a resistance that performed the heating.

OK, if you want a time proportional control it means that in the period T the relay must be some time ( t1 ) ON and T-t1 OFF.

When you start the control, if SP is greater than ambient temperature, it will be 99% ON and 1% OFF. Approaching to the SP it will be ( just as an example) 60% ON and 40% OFF.
If the control and the system is dimension correctly when the process variable ( temperature measure) is equal the SP the relay should be 50% ON and 50% OFF.
Is important to notice that in the period T you should have both state.
For some commercial oven and boilers 60 sec is a good T.

I hope this comment will be useful.

Good Luck
Daniel