Regarding H1B visa extension

Hi All - My H1B visa got expired on Dec 2014, i have applied for extension on Nov 2014 through my company and got query from USCIS in March 2015 and submitted the required doc. As per USCIS update now the vermount is verificaing the Documents and as per document submit on April 2015. Till now didn’t get any other updates on it.

Kindly let me know how long it will in these cases and if petition approved from which date (start date) the visa gets extension.