What does “stable” mean?
For a process to be implemented on a workstation, it should not change for at least 6 months.
For a process to be implemented on a small integrated system, it should not change for at least 1 year.
For a process to be implemented on a large scale integrated system or on custom hardware, it should not change for at least 2 years (3 is even better).
Violating the above rules usually means that there is much less (and sometimes zero or worse) realized economic advantage to automating.