Exactly what everyone here has already replied...

Treatment plans are best determined in conjunction with the client, asking them what kinds of things they would like to get out of therapy. The professional often takes the ideas expressed by the client and puts them into measurable and achievable chunks.

So, for instance, let's say your goal is basically to not have PTSD any more, it might be expressed in getting rid of one or more of the most harmful symptoms. For a lot of people with PTSD, that might be reducing the number of flashbacks experienced in one month from X to Z number. Sometimes that Z number might be zero, but I found it's also helpful to be realistic in treatment plans, so just reducing the number of flashbacks from X to X-2 per month or something is often more realistic.

Then such goals are more easily measured because the therapist should ask (at least once a month, but ideally, every session), "How many flashbacks have you had this week?" If you go from 2 to 1 every week after a few months of treatment, then boom, you've achieved that goal! Then you set a new goal, until you actually do hit zero a month (or something like once a year... even when a person is done treatment for something like PTSD, it doesn't guarantee you'll never have another flashback).

Treatment plans, in my mind, should always be realistic and achievable within a few months' time. They should also be revisited on a regular basis to ensure that psychotherapy is actually doing something for the client. Otherwise, you have no semi-objective way of knowing whether any of that is working for you!

Best,
DocJohn