No manufacturer that I am aware of specifies a "pre-heat" time. In fact, if the system is certificated as a primary automatic system, it will be activated by an ice detector, which, obviously, has to detect ice first.
That said, it can take a finite period of time to heat the wing to a fully evaporative state, perhaps a couple of minutes ( assuming it was designed to be fully evaporative). This would completely depend on the hi-stage air temperature and volume flow, size of the wing, wing temperature, RAT, and mass of liquid water being encountered.
When I flew the MD-80, I would routinely heat up the main wing while being vectored above the clouds if I expected ice during the approach. This was simply because the tail had to be cycled before beginning the approach, during which time the main wing would not be getting any heat...so I wanted the wing hot to begin with. I really don't know if that worked the way I thought it would or not, since I have no idea how fast the wing cooled once the air was shut off, but it seemed like it was a good plan.