How long is a winds and temperatures aloft forecast valid for?
Advisory circular 00-45 "Aviation Weather Services" documents the winds and temperature aloft (FB) product on page 7-39.  Table 7-11 lists the standard forecast periods for these products.  



From this table you can see that the 6-hour forecasts are usable from the time they are issued until 3 hours after the valid time.  The 12 hour forecasts are usable from -3 to +6 hours of the valid time and 24 hour forecasts are usable from -6 to +6 hours of the valid time. The products are issued 4 times daily.

While these are standard times, you should consult the specific information available in the product header that has this information.  For example:

DATA BASED ON 010000Z
VALID 010600Z FOR USE 0500-0900Z. TEMPS NEG ABV 24000
FT 3000 6000 9000 12000 18000 24000 30000 34000 39000
MKC 9900 1709+06 2018+00 2130-06 2242-18 2361-30 247242 258848 550252


This says the product is valid at 06Z and for use from 05Z to 09Z and the data is based upon the 00Z forecast model run.