3 months in advance.

Please refer to my postin on 12/20 regarding the required documents. Your H1B extension process is the same as your first H1B. Meaning you need to go through them once again starting from LCA. The only difference is there is no quota requirement for H1B extension.

Normally, you file for H1B extension 3 months in adavnce is a reasonable timing, otherwise they won;t accept it. If you are in a hurry, you can file premium processing for extra $1000. You should get response within 15 days.