From a practical point of view, the counter starts when the VOP request is initiated by the requester and materialized via the VOP request Timestamp (attribute AT-T056). It is then up to each scheme participant to agree with its RVM(s) how manage this at each step of the process.
From a merely legal perspective, as the timeline is defined in the VOP scheme Rulebook and the sole parties to the Rulebook are the PSPs (and not the RVMs), the scheme participant PSPs remain responsible for the whole execution timeline.
This VOP Request Timestamp must be compared with the VOP response timestamp (included in the response header). The difference between the two must be less than the maximum execution time (5 seconds).