How long I need to work for old employer befor I can do h1b tran

I changed my job two month ago and the h-1b transfer is already approved. Now I got another job offer so I need transfer my h1b again. I heard that you have to work for employer for substantial time of period like 3-6 months then you can transfer h1b. Is that true I have to wait that long to transfer? Thanks for the input. by the way, do I need to get receipt to work for new employer or I can work as soon as new employer filed petion for me (for example, fedex tracking shows the file sent out).