Hi!
I think you are OK. In a normal cycle, her ovulation should occur around June 28-July 2 or so, and since sperm only surive for 3 days, even if you had not used any protection, you would probably be OK. One thing to note: Plan B works by delaying ovulation, so don't be surprised if your wife's period is a week late. As long as it is normal and comes within 1-2 weeks of the normally expected time you are probably OK. If her period is more than 2 weeks late, then it would be time to check a pregnancy test.
You may want to give some consideration to a more reliable long-term solution, Mother Nature has her own ideas about when to send the stork!
Good luck!
Dr B