Trust was for children who didn't realize yet what they had to lose and that description fit so few people in Slytherin House. Certainly not Snape or Regulus Black.
But they had been friendly once, and that deserved...something, perhaps, since not that many people bothered with things like that when it came to Severus.
"For me it's been nearly ten years," Snape said, shrugging the hurt tone off as if he knew it was an act. And, well, Severus taught children for a living, it wasn't so difficult to see these days. "So, rather more than a few weeks."