I have a question about the last sentence in this paragraph:
“Ecce omnem fabulam habes de puero temerario qui libertatem quaerens mortem invenit. Iam tempus dormiendi est. Nonne fessus es longas fabulas audiendo?”
Here is my translation, and please correct me if I am wrong:
There you have the whole tale of the foolhardy boy who (by) seeking freedom found death. Now it’s time to sleep. Aren’t you tired of hearing long stories?
My question concerns the gerund audiendo at the end of the last sentence. Is it a dative or an ablative? In English it felt quite natural to translate it as “of hearing”, i.e. an ablative, but maybe Latin is different. I’ve come across some pretty strange datives and genitives in my studies so far and maybe this is one of them? 
“Tired of hearing” suggests that Quintus is bored. But I think the question is literally are you tired by the hearing of long tales, that is are you worn out by the length of the tales. The slave telling the story is concerned about the boy actually becoming tired. Often in English we say “oh I am tired of hearing about that” when we have only heard someone open a discussion on a tiresome issue. We are hardly fatigued in a literal sense.
Quintus’ reply “Non sum fessus, nec illa fabula longa esse mihi videtur” would seem to suggest that he found the story so compelling that it didn’t seem long at all and that he was not tired.
I think it is natural to understand an ablative here, and translate "“by hearing”.
Also, I think your parenthetical “by” is unnecessary.
Thank you so much seneca2008 for your excellent response. “By hearing” is of course a much better alternative for the reason you mentioned. I think the problem here was that I was thinking in Swedish while translating into English (“av att höra” = “of hearing”).