A/THR is throughout the flight:
technically, when we take off, the A/THR is 'armed' with the thrust levers in TOGA or FLX setting, when we bring the thr lvrs back to Climb power at about 1500' agl then the A/THR become 'engaged' and speed and thrust setting are controlled via the FCU/FMGS.
The position of the thr lvrs really just tells the system the MAXIMUM thrust that it is allowed to apply, if we bring the thr lvrs back w/o pressing the little red buttons on the side then A/THR is still engaged but is limited to whatever thrust setting the levers are at.
'course, you can always just disengge it and use the thr lvrs like normal (dare I say it..) 'throttles' (ugh!)
It is disengaged once the thr lvrs are brought back to idle on landing. putting the thr lvrs back to FLX or TOGA re-arms the A/THR.
Basically.