I don't have a rule of thumb, but from my experience, if you just place your pot of wort in a cold ice water bath, occassionally stir the ice water, and add more ice to the water bath to keep it cold, by the 45 minute mark it should be below 80F. That' was my experience with about 3 gallons of wort.
If you really have a lot of ice and really keep the water bath cold, and occasionally stir the wort itself (with a sanitized spoon, of course), you can significantly speed it up.
Anyway, there is no real hurry so long as all conditions are sanitary. I think it is better to wait a little extra time to be sure the wort is not too hot, than to rush it.