On tickets for the ascent and descent, only the ascent time is reserved. The descent session is not reserved. So, once you have reached the limit of your time at the upper station, all you need to do to make the descent as included in your return ticket, is to join the queue for the descent, and descend in order of arrival.
If your question relates to your ticket for descent only, because you intend to make the ascent to the peak of Mount Teide on foot, in accordance with the new regulation relating to permission for access to trails PNT 7+11 “Montaña Blanca – Mirador de la Fortaleza – La Rambleta”, PNT 9 “Teide – Pico Viejo – Mirador de Las Narices del Teide” and PNT 23 “Los Regatones Negros”, as well as the section of Trail 28 (Chafarí), and in accordance with the measures implemented by the Island Council Office for the Natural Environment, Sustainability, Safety and Emergencies, a ticket for descent only can be booked once at the upper station using QR codes, and providing that the cable car is in operation. That is why it is essential to allow for the possibility that, due to adverse weather conditions, technical issues or other problems caused by force majeure, the cable car cannot operate, making it necessary to make the descent on foot.