After changing a display name, there is a 7 day grace period after the 28 days are up in which the username will still be unavailable to others. This means after 35 days, if you decide not to revert back to the previous display name, it will be up for grabs. This is the safest way as you're able to calculate and pinpoint at what particular time the name will be available to grab on a different account.
The other method would be to change your name twice. This can be done with bonds.