Time gap betweenH1 visa extension application and approval

What is the normal time between application of H1 visa extension date and final I797 approval receipt? Also is there something like minimum period and maximum period in such cases ?

Under normal processing, H-1 extension can take 2-6 months to process. If filed w/ premium processing, then it will be adjudicated within 15 calendar days. If RFE is issued in either case, then it can take longer.