SRO tickets are $42 each, payable by cash, debit, or credit. You can buy SRO tickets when the box office opens if the show you want to buy tickets for is sold out at that time (which it oftentimes is). It's very difficult to gauge when to get in line, as the situation is different every day, but based on what I've read (and experienced), being in line 3 hours before the box office opens on cold, two-show weekend days (which, in your case, is 7am) will probably be safe. Though, there are never any guarantees (which is why you often see people lining up 5+ hours beforehand). People tend to arrive at 15 minute intervals, so if you get there at an "in-between time" (e.g., 650am instead of 7am) you may beat out groups of people; but this is more of a guideline rather than a rule.
I would encourage you to read through this and other threads on Dear Evan Hansen to make your best judgment. Just remember that every day is different. Some days you can get to the box office at 630am and not get a SRO ticket; other days you can walk up to the box office 5 minutes before showtime and snag a SRO ticket (this happened to the person standing next to me at this past Sunday's matinee).
Updated On: 2/21/17 at 10:40 PM