The DOS has gotten very efficient with the NVC and their goal is to move cases on, they don't want to sit on hundreds of cases. You can play a dance by extending the one year termination by delaying at certain stages complying with NVC requests. But this is a very delicate dance and needs you constant attention or the petition can be terminated.
Consequently, you have one year after being notified by the NVC that your priority date is current and you should apply for the visa. Failure to apply within the one year will trigger the NVC to return the petition to the USCIS. At that point the petition would be terminated.
"The Secretary of State shall terminate the registration of any alien who fails to apply for an immigrant visa within one year following notification to the alien of the availability of such visa, but the Secretary shall reinstate the registration of any such alien who establishes within two years following the date of notification of the availability of such visa that such failure to apply was due to circumstances beyond the alien’s control.
State Department regulations at 22 CFR § 42.83 provide further instruction to the agency on how to proceed when the visa applicant has failed to appear at the consular interview, to apply for an immigrant visa after notification by the National Visa Center (NVC), or to present evidence overcoming the basis for a consular refusal. The alien is given one year to re-schedule the consular interview, respond to the NVC notification, or submit the additional evidence. If the alien fails to do that, the DOS is empowered to terminate the registration and send the alien notification. The notice also informs the alien that the registration may be reinstated if, within one additional year, he or she establishes to the satisfaction of the agency that the failure to apply for the immigrant visa, attend the interview, or submit the necessary evidence within the first year was due to circumstances beyond his or her control. Examples of such circumstances include “an illness or other physical disability preventing the alien from traveling, a refusal by the authorities of the country of an alien's residence to grant the alien permission to depart as an immigrant, and foreign military service.” Failure to receive the DOS notification of the availability of the visa or of the consular interview is not a valid reason if the applicant did not keep the DOS informed of his or her address.