My I-140 is approved and H1B extension is in progress. My priority date is April 2013 (EB2 category).
My question is, How long it will take approximately for my priority date to be current?
And I beleive I will have to wait for priority date to be current to apply I-485?
Yes you will need to wait until it is “current” in order to apply for your I-485. To understand how long it will take to be current, you can see the chart on Current Priority Date as it shows historically how the priority dates have moved. You can also subscribe to get notified by email when your priority date becomes current and find other information such as what it means for your priority date to become current.