I think we have to question if Onada really still thought in days, weeks or years. At least to me, it does not sound totally unbelieveable that he eventually stopped counting days and weeks and rather lived a sequence of days. A very long sequence of days, long enough to make him lose track how long it was.
At least to me, this is kind of plausible, because he had no more reference in his days. You know: Hey, every seven days there is this special day where I cannot go shopping if I need something. This coincides with a week going by, and thus, you have a fairly "natural" notion of a week. For him, I guess, a day consisted of getting up, checking his surroundings, gathering intelligence, staying hidden, maintaining his weapons and eventually hiding and going to sleep again. Day after day after day.
At least to me, this is kind of plausible, because he had no more reference in his days. You know: Hey, every seven days there is this special day where I cannot go shopping if I need something. This coincides with a week going by, and thus, you have a fairly "natural" notion of a week. For him, I guess, a day consisted of getting up, checking his surroundings, gathering intelligence, staying hidden, maintaining his weapons and eventually hiding and going to sleep again. Day after day after day.