I've been a clear member for about a year.
The screening part of the application process took months - 3-4 total, I think.
In my year of membership Clear has saved me only about 40 minutes total - most of that came from two flights out of SJC when flying Southwest. In one of those cases I got through early enough to get on an earlier flight so that was well worth it.
In most cases where I have used it at JFK, RNO, IND - I have only shaved a few minutes off the security process.
The TSA security people aren't always 100% cooperative with their part of the process which reduces efficacy to some degree.