We don't really know how long an octopus can survive without taking in food from the environment (and it is likely species dependent). There is a recent case that suggests one of the deep water animals broods for as long as 5 years. The observations had long timelapses and feeding behavior was not part of the observation (I still have discomfort at the idea of a 5 year brood). The GPO is known to brood for 6-10 months and thought not to take in food over the entire incubation period. There are also observations of brooding animals continuing to eat but all lose muscle mass and metabolize their own tissue. The muscle wasting is quite noticeable in aquarium kept animals for the longer brooding animals (like the GPO and O. briareus) but not as evident for the animals that only brood for a couple of weeks (a. aculeatus).
Do you still have fish in the aquarium?
Do you still have fish in the aquarium?
My guess is old age but (and I have had this happen as well) you would normally expect eggs before death. I would check her den (Edit - I posted this while you were posting your, "no sign of eggs" post). Small egg species do not brood for long (any where from a week to two weeks has been recorded). The other animals may have already eaten the eggs but if not, they will likely be visible. I removed Little Bit's eggs and placed them in a dish of water to see how long it would take them to disintegrate. I forgot to add water several times so the salt may likely preserved them much longer than is natural but they looked the same after several months before I finally disposed of them (they were known to be infertile).
