Difference between l1B visa duration and passport stamp duration and extension rules

Hi all,
My L1B visa was granted for 3 years, however, I received a stamp in the passport only for 2 years.
Do you know how can I extend it?
1. Can I do it within USA?
2. If not, should it be my home country or I can apply from any country where there is USA visa consulate?
3. How long it takes in current conditions?