The block heater will reach the best (optimal) temperature in about 1 1/2 hours. If it is very cold, it will warm the engine block just warm to the touch. If it is 32F, the engine will feel a lot warmer. In any case the timer need not be on longer than 1 1/2 hours. I have used timers for 40 years, and have worn out 4 of them. Block heaters have dual benefits; they allow easy starting with reduced engine, battery and starter wear, and in addition they save gas. The little bit of electricity used has a big payoff. For some people, getting heat in the car quickly is also a major benefit.