Trekking in Sikkim?

Has anyone booked a Sikkim trek whilst in Gangtok, Darjeeling, or Calcutta? If so how long did you have to hang out before the trek started?

Any other info would be welcome.

Cheers.

#1...I have noticed that all of your posts contain an advertisement for your company. It must be great to use Lonely Planet for free advertising.

Also the info #1 provided is misleading. It is the best way only because his agency is based there - it says a lot about his&his company ethics. So we all know whom to avoid...
In fact there are plenty of good agencies based in Gangtok which can arrange trek on a short notice (a day or two).