Tekashi could be back on the streets by early August based on the good behavior guidelines established by the Bureau of Prisons ... according to his lawyer, Lance Lazzaro, who's taken a closer look at the numbers.

6ix9ine got 24 months, but with 13 months credit for time served ... leaving him with just 11 months remaining. That would get him out around November 2020, but then the BOP math kicks in, and the potential sentence starts shrinking.

BOP guidelines call for up to 54 days per year to be shaved off IF inmates show good behavior. So, with the 2-year sentence, he's eligible for up to a 108-day discount.

Assuming Tekashi is as model an inmate as he was a rat ... Lazzaro says the rapper could be out by August 2020.

As we reported ... the judge in Tekashi's case said the rapper cooperated in a tremendous fashion, but his conduct was much too violent and selfish to get off with just time served.

Still, he was staring down a much longer sentence -- almost 25 times more than what he got -- and if he minds his Ps and Qs on the inside, he'll be out in time to drop a summer single.

"Gummo Pt. 2" anyone? Probably minus the guns and gang members this time.