What is the time period for a person to be on a ventilator for copd?

Varies... Disease progression varies widely among individuals and there are many factors that affect the rate of progression. In general, the disease will progress slower if you stop smoking and use oxygen, if the oxygen level in your blood is low enough to warrant this therapy.
More info needed. If someone is on the ventilator, the time it takes to get them off of it is highly variable and individual. The reason they ended on the ventilator is a big part of the answer, together with their age, the severity of the illness, what other things are going on ...Etc.
Depends. Depends on how bad the COPD is and what the patient's wishes are. If the COPD is really bad, some patients may need to be on the vent for life unless they opt out of it. If there is something reversible such as a pneumonia or congestive heart failure exacerbation, would be reasonable to wait until that is treated before making decisions.