When there are no more suitably qualified internal candidates for upgrade. CUrrently there are 100+ SFOs on the command course waiting list, with no significant fleet expansion planned for a long time. Even more so in the current economic climate.
Perhaps EZY will never again need DECs - we may never again see the rapid expansion of the last few years.
A very long time, if ever; unless you get a management job or are a TRI/TRE.