How do we know when to activate flow freezing and which type of flow freezing (periodic or permanent) should we use in Solidworks flow simulation to speed up time dependent calculations ?? Is flow freezing only way by which we can speed up calculations ?? And what is probability that results which we will get are close to accurate ?

Flow freezing holds the convective terms constant with the intent to allow the diffusive processes to catch up. Consider a desk top computer, you turn it on and the convective flow of the fans and what not achieves quasi steady state in low numbers of seconds. The steady state temperatures will take orders of magnitude longer. If you think about that it will tell when you can use it and when its not an appropriate option. The big hazard is turning it on too early. The convective terms need to be in a quasi steady state before you turn it on. You can check this by rerunning the problem and say starting at 0.5 travels instead of the default 0.25 travels or run it without using flow freezing. If the convective terms are going to change as say the temperature changes then the periodic option is a good choice to capture those effects.