my suggestion: drink six cups straight
I let a cup of green tea stand for two days once, and it was already visibly moldy after that time. I mean, it doesn't have to be toxic right away, but it can be and you won't see the mold right away... when its furry, its already an advanced stage.
I love Fu Cha
which is super moldy, but thats the right kind of mold
I guess it depends on the person, and also on how frequently you do that.
For me, the best way is to finish the leaves in one session, and the second best to do is to start a tea in the morning and finish it in the afternoon, and beyond that I don't go
If a tea has a lot of stamina, I tend to use less leaves, only two to three grams, that usually requires longer infusions and will empty the tea leaves faster.
I'm talking mostly about Oolongs and White tea here, especially Yin Zhen can last very long if the quality is good.
But decades-old Pu may be another dimenstion, I don't know about these.