It's probably best not to discuss it in terms of "leaving the Earth's atmosphere". The shuttle never really did, the atmosphere in some form goes out very far indeed.
However we can look at some times, velocities and burns.
For STS-135 these are the important times (given in minutes and seconds from liftoff)
- Solid rocket booster staging: 2:03
- Main engine cut-off: 8:23.8
- OMS-2 perigee-raising burn: 37:45
And the corresponding velocities
- Main engine cut-off: 25817 ft/s
- OMS-2 delta-v: 96.8 ft/s
Source: Space Shuttle Mission Summary