What return period is used by ASCE 7 for Wind Speed, is a complicated question. The reason it is so complicated is because ASCE 7 has changed the basis for the wind speed over the years. Here is what is used starting from the oldest to most recent versions of the standard:
ASCE 7-88 & ANSI A58.1 & UBC 97:
The ASCE 7-88 & earlier used Fastest mile wind speed. To convert between the two we need to do the following calculation.
(3600 sec/hr) / (Fastest Mile Wind Speed) = Time
Then use that Time to convert to 3 sec gust (Or whatever basis you are using) and get equivalent speed.
ASCE 7-95 to ASCE 7-05:
Basic wind speeds assume 50 year return (Probability 0.02), 3 second gust.
Importance factors were used to effectively convert Category III and IV to a 100 year return.
ASCE 7-10:
Basic wind speed was now dependent upon the Category, and the return period was based upon the following categories. The wind was still a 3 second gust.
Category I - 300 year return
Category II - 700 year return (Probability 0.0014)
Category III and IV - 1,700 year return (Probability 0.000588)
ASCE 7-16:
Basic Wind Speed was still dependent upon the Category, and Category III and IV were no longer combined. The wind was still a 3 second gust.
