Haughtney the 120/180/240 time is regularly exceeded. The TAS achieved in the critical fuel configuration is most often considerably slower than that used to establish the range circle or area of operation.
For example, the approved TAS for establishing range circles up to 180 mins on a B777-300 is approx 420 kts established at the nominated single engine cruise TAS.
The Critical Fuel scenario is normally ETP1D (single engine depressurised at FL140). The TAS in this configuration is considerably slower than 420kts
This is the most common reason for long diversion times.
As you say wind can also be a factor for exceeding the time.