Wycombe
At least someone gave it a thought!!
The longest (in terms of Great Circle Distance at 908 nm / 1045 sm / 1682 km) is ARN - GVA with SAS with a Dash8-400. Block time is upto 3 hours 15 minutes.
If you think it should be measured in block time then the longest is CPH - MPL (GCD = 795 nm / 915 sm / 1473 km) which on a Cimber ATR72 is scheduled at 3 hours 50 minutes.
By way of contrast SOU - GVA (in terms of GCD) is only 409 nm / 471 sm / 758 km I would guess this would be around 1 hour 50 minutes.
Source: OAG MAX Feb 03