h1b will expire 09/06, when will be deadline to file h1b transfer before h1b expire

[jane2016]

my first h1b will expire in 09/06, I just filed h1b extension. But for some reason, I want to change job asap. don't want to wait for h1b extension to be approved before changing job.
Could anyone let me know when will be the deadline to get offer and file h1b transfer? Do I have to get the h1b transfer receipt before 09/06 or I just need to file h1b transfer before 09/06?

thanks a lot

Alice