It depends how early into a new methodology you are. That said, I think once you have a solid basis, you should make no more than one change in your training on a yearly basis. This can be a volume increase, a new type of workout, a schedule change, etc. Early on you can do this every 3-6 months, but once again only make one change at a time. If you find yourself needing to change more than that, you either need that reminder to "trust the process", or you need to start from scratch because you made some major mistakes early on.
As far as metrics: generally the only metrics I consider are (in order of importance): health, race times, and enjoyment. Time in the market beats timing the market, and this is true for running as well; it's a long journey to the top, and there are no shortcuts. Staying injury-free is the key to making that happen. Being process-oriented is vital, but there is certainly a place for results as a validation tool. If I run 5 seconds slower per mile in every race distance from one year to the next, then ceteris paribus, my training plan got worse. This also leads into the last metric, because getting worse at running sucks. Getting better feels good. Enjoying the daily training is important too. This is one of the reasons I love double T so far - the workouts are a blast because you feel so fit and strong, you're only really straining once a week, but you still get the satisfaction of putting in huge workloads (20+ mile days twice a week every week). Most enjoyable and effective training I have tried yet!