I agree with @Spdracer39 that it really depends on the total duration you need to ride that day.

It also depends very much on your level of conditioning and training, and how much recovery you'll get between the climbs, and your genetics.

The key is to keep it at or below threshold, in which case you can keep riding for a long time. Time spent over threshold will decrease the total amount of climbing you can do that day (but of course will result in a faster time).

For me personally, if I stick to 80-85% FTP on the actual climbs, I can keep going semi-indefinitely. Hours (like 6 hours of up and 2 hours of down/stopped time) would not be a problem. If I push things to 90-100% FTP, its going to be way less, like a couple of hours. Despite the definition of FTP, I don't think its true that everybody can only ride at FTP for 1 hour. But that conversation is beyond the scope of this post.