The forecast winds tend to come from the flight plan, for FL300, 200 and 100 and are entered into the FMC prior to the descent.
The time of the forecast used varies , but for flight plans printed say 1 hour prior to departure, by he last sector of a long day I imagine the forecast could be up to 12 or 18 hours out of date depending on the system and the model used.
In the 737NG, any direct routing executed on the descent automatically inputs the present sensed wind into the FMC calculation process.