User blog comment:B214/Volume 21 info & spoilers/@comment-191.82.149.185-20160704214534/@comment-93.105.241.146-20160706210707

The highest boost was when he got really super pissed off on Vali when Azazel told him he will shrink Rias' boobs. He boosted 41 times back then so 2^X-1 (yes thats the proper formula as the first boost is the one that brings his power to 2) means 2^40 = 1099511627776.

So he became over 1 trillion times stronger. However you have to take into account it wasnt his real ballance breaker at the time. So wether or not his strength was boosted so much is dabatable.