In some ways it's kind of a dumb question, right? You want to run as far under your standard as you possibly can, both because that gives you the best chance of running the race, but also because that's why you run a race.
I mean, if we told you to run six minutes under and you set that as a goal, and reached it, you'd be a better runner than if we told you to run two minutes under and you set that as a goal, and reached it; but you'd have to not blow up. So really, only you know what you're capable of, and the goal is to run as far under the standard as you can do.
Nobody can promise a specific time will get you in - you're literally racing against everybody else who's trying to run Boston next year, with the prize going to the ~20,000 who beat their standard by the largest margin.
But to throw some numbers out there, you're obviously safe if you beat it by ten minutes. That also gives you the security of likely getting your confirmation before the registration even ends. Five minutes under gets you registration during the first week, and so far they've never had to shut it down during the first week since they went to the lottery.
You're not safe if you have to register the second week, but realistically three minutes under should put you in a decent spot. Any less than that, and you'll be sweating it.
But seriously, beat it by as large a margin as you possibly can. It's a race. Don't say, "well, I'll be in with three minutes to spare so I should relax" with three miles left in your race.