Learn English – What does “day’s” mean

"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.

Thesis is singular, theses is plural. Thesis' is singular genitive, theses' is plural genitive. As a general rule for genitives of words ending in s, you use s', the Saxon genitive.

So, assuming you want to talk about the multiple goals in multiple theses, then you would use "theses' goals". If you want to talk about multiple goals on a single thesis, you would use "thesis' goals."

As a general rule you can translate "x of the y" into "y's x", these being the two forms of genitive in English. However, beware, there are many instances where the two are not interchangeable.