Without getting into it too deep a 100 gram decrease in drag will save you:
30 secs @ 30mph over 25 miles
40 secs @ 25mph over 25 miles
50 secs @ 20mph over 25 miles
70 secs @ 15mph over 25 miles
A typical 36 spoke wheels has a drag of 260+- grams while a dsik wheel has a drag of 110+- so thers a 150g difference between the best and worst of wheels. In a nutshell...its not going to save you much....
BTW...the above chart is correct...I cant explain why you save more time at lower speeds...I only copied it from the book.