The Gangtok run follows the Teesta most of the way and is one of the most straightforward hill drives in the region, but it is long. Fares are quoted for the whole route so a slow stretch does not change what you pay.
If you need an Inner Line Permit, the driver knows where the checkpoint is and how long it takes. Build in the extra half hour.
Permits and checkpoints
Indian nationals do not need a permit for Gangtok itself. Foreign nationals need an Inner Line Permit, issued at Rangpo on the way in — carry a passport, visa and passport photographs.