Depending on how long it takes in the department or immigration office in your university (sometimes it takes months before they submit the files to USCIS). Then USCIS may take 2+ months too; but it can be expedited by paying premium processing. If this is a new application (such as from F-1 to H1B), you cannot work until it is approved (if it is H1B extension or H1B transfer, you can work with a receipt).