i was taught to that make sure you have two minutes to get a medium/small off behind a 757 as we have all been caught out by pushing a gap for a minute departure and when the plane lines up the captain requests the full 120 secs.
In practice ( if there is nothing on final), after one minute, we tend to say WHEN READY clear take off, in my experience about 70% will go after a minute, about 25% after 2 minutes and some take more - i have seen 4 minutes.
The main thing is that all we have to give you in GB is 1 minute - if you want more - tell us BEFORE YOU LINE UP - we will be very happy to give you more time.
Just a point, we time our 120 secs so that the planes rotate 120 secs apart - if a pilot is timing it (ie has asked atc for 2 minutes - we say roger report rolling) it always seems to be about 3 mins, it seems that they count 120 secs from when the previous departure has rotated THEN the second arcraft starts to roll. I have had a pilot tell me that he had not had the full 120secs - he had but i think he was timing it this way. - Any comments??? (the first a/c wasn't a 757!)