I-551 (Temporary card) - Please help

[muralip]

Friends,

My I-485 was approved 6 months ago and not yet received my green card. Planning to to go India in Feb 1st week and planning to get I-551 stamped using InfoPass.

a. Usually how long it's valid?
b. Do I need a transit visa if my flight stops at London (with I-551)?
c. What documentation do they ask?

Pl answer and your help is appreciated.

thanks
Manish