Understanding the role of mergers in galaxy formation is one of the most outstanding problems in extragalactic astronomy .
While we now have an idea for how the merger fraction evolves at redshifts z < 3 , converting this merger fraction into merger rates , and therefore how many mergers an average galaxy undergoes during its history , is still uncertain .
The main reason for this is that the inferred number of mergers depends highly upon the time-scale observational methods are sensitive for finding ongoing or past mergers .
While there are several theoretical and model based estimates of merger times , there is currently no empirical measure of this time-scale .
We present the first observationally based measurement of merger times utilising the observed decline in the galaxy major merger fraction at z < 1.2 based on > 20 , 000 galaxies in the Extended Groth Strip and COSMOS surveys .
Using a new methodology described in this paper , we are able to determine how long a galaxy remains identifiable as a merging system within the CAS system .
We find a maximum CAS major merger time-scale of 1.1 \pm 0.3 Gyr at z < 1.2 , and a most likely CAS merger time-scale of 0.6 \pm 0.3 Gyr , in good agreement with results from N-body simulations .
Utilizing this time-scale we are able to measure the number of major mergers galaxies with masses M _ { * } > 10 ^ { 10 } M _ { \odot } undergo at z < 1.2 , with a total number N _ { m } = 0.90 _ { -0.23 } ^ { +0.44 } .
We further show that this time-scale is inconsistent with a star formation origin for ultra-high asymmetries , thereby providing further evidence that structural methods are able to locate mostly merging galaxies .