If you're talking about simply letting a name expire, it all depends on how long the name has been on the account it's expiring off of, for instance a 10 year old original login name can take months and months to expire, but a random fresh account only having held the name for a few weeks prior to it being changed could have the name released after 36 days or so
No name will expire from an account(get released) less than 35 days from the original change.
There is no way of telling what time of day a name becomes available if you're waiting for it to expire.
If you've bought a name it will be available to change for a second time(it won't randomly get released to everyone) when the 28 days or so is up. It won't get released to everyone for at LEAST 35 days from the first change.