My H1B and my wifes H4 visa are sent to USCIS for extension. We have received the receipt, but the same is yet not approved.

Now we have a situation- My wife has to leave the country to attend her brothers wedding in India. We wanted to know -

1. Can she leave the country based on USCIS receipt awaiting approval as she doesnt intend to be back for few months i.e can wait till approval.
2. When can she re-enter? (I understand that once H4 extension is approved she will visit the local US embassey and get new H4 stamp).
3. How long does it normally take for approval to come.

I am on H1B and I dont intend to leave the country till approval. It is all about her i.e. H4 visa.