What does "a day's work" mean?

What does "a day's work" mean? Does it mean a full day's work?

The quote below is taken from Charles Dickens' The Haunted House

In the summertime, I often rise very early, and repair to my room to do a day’s work before breakfast.

"A day's work" typically means the amount of work one would normally do over the course of a day.

There is no objective measure of this as the amount of work would vary by person and by job.

For most people, though, it would not be possible to do "a day's work" between waking and breakfast, so in the Dickens quote the phrase is probably being used for literary effect.

In the context of the quote, it could be hyperbole. The speaker may not actually be doing the same amount of work before breakfast that he would literally do over the course of a day. Rather, he may be exaggerating to illustrate how early he rises or how productive he is in the early morning. On the other hand, it is also possible that the phrase is being used in the quote for the opposite effect -- to illustrate how little work the speaker normally does in a day, i.e. he does so little work normally that he can accomplish "a day's work" before breakfast. It's hard to determine the intent of the phrase in this quote without more context.