Depends! Vortex separation has to be 120 seconds (2mins) or 180 seconds (3mins) as appropriate. However, vortices are not considered to begin until the aircraft rotates, so we're trying to apply 120 ( or 180 ) seconds from rotate of aircraft 1 to rotate of aircraft 2. We normally allow about 40 seconds for the take-off roll (30 secs if the aircraft is particularly light or there's a good headwind), so I can clear aircraft 2 for take-off approximately 1min 20 to 1min 30 after aircraft 1 rotates.
Who said Air Traffic wasn't an art form!
Personally I use the ATM (radar) and I clear aircraft 2 for take-off when aircraft 1 is 4 miles on the climbout on a calm day and 3 to 3.5 miles when there's a headwind. This guarantees me 120 seconds vortex separation and is easier than trying to subtract 1m30secs from 1354h14secs when I'm busy.