Need help with setting timelines to Nursing Outcomes

How do I know what an appropriate timeline would be for setting a patient's desired outcomes, in particular in an outpatient setting (with a fictitious patient). Seems like it's easier with patients admitted to the hospital.

For example, if my desired outcome is for this patient to quit smoking, how do I know what an appropriate timeline would be for this? It could take some people weeks to quit, but others it could take years. If this was a real patient, I'd include them on setting the outcomes and ask them what an appropriate timeline would be for them, but this is a fictional patient. Do I just make up a time? Can I say "patient will quit smoking within 6 months"?

Thanks to anyone that can provide a little clarification!