Generally, the pass report is good for 12 months (or 1 year). 

However, a pass report used for an ownership transfer can be used to renew a vehicle’s registration at any point during the calendar year that follows the test.

There ya have it (depending on who you believe ).

You don't need another E-test as long as you buy the stickers before one year has passed since the test was done.

Here's something else that might apply .... the last car my wife bought (used) did not come with any emission test paperwork but we knew, based on the sellers birth date, the year of the car, and that it was presently licenced and stickered, that it had to have had a test (and passed) within the last twelve months.  Sure enough, we went to transfer and plate the car (without any emission paperwork) and the woman at the counter said that according to their computers, the car was up-to-date emissions wise.