The responses are best to my knowledge
1. How can i get the recipt that DOL received my application?
A) May be ur attorney or employer have received some thing,Check with them.
2. What all do i need to apply for 7th year extension?
A) all documents for a normal H1B and Copy of any kind of proof for ur pending labour,may be u can get this from ur SESA ,pls check with them
3. When can i apply? Can i apply in advance or do i have to wait for 6 yrs to get over?
A month before your sixth year ,the only disadvantage is u need to pay same amt of fee 1130 i guess for 1 year
4. How much time does it take for the 7th year extension?
A) Its a normal procees of a h-1b in the respsective Servive center unless u go for premium processing !!
Good Luck