The average woman is on HRT for 5 years, some more some less of course. It is safe to be on HRT for as long as needed to treat symptoms. Winona generally suggests a trial off the medications after 2-3 years to see how the patient feels, if they feel fine off, it is time to stop. If the symptoms return the medication can be restarted. This can then be repeated every year or so.
HRT can be stopped at any time with no adverse consequences. Most women feel better on HRT so they stay on for several years, which is safe. But if you decide it isn't for you, stopping the medication is perfectly safe.