H1b - Approval notice

1) How long does it take to receive the approval notice? My uscis account shows that my case was approved in last week of February. But the attorneys still don’t have the approval notice. 2) Does the time required to receive the approval notice depend on premium vs regular processing? 3) Is it safe to resign before receiving the approval notice? I guess I wouldn’t know how long my i797 is approved for? (3 years or lesser)