Nie mam kiedy odpocząć, bo w tygodniu zawsze pracuję do szóstej.

This is a very common Polish structure: mieć + question word + infinitive.

So:

• nie mam kiedy odpocząć = literally I don't have when to rest
• natural English meaning = I don't have time / a chance to rest

Polish often uses this pattern with different question words:

• Nie mam gdzie usiąść. = I have nowhere to sit.
• Nie mam z kim pójść. = I have nobody to go with.
• Nie mam czym zapłacić. = I have nothing to pay with.
• Nie mam kiedy odpocząć. = I have no time/chance to rest.

So even though it sounds unusual from an English point of view, it is completely normal in Polish.

Yes, you could, and it would also mean something like I don't have time to rest.

The difference is subtle:

• Nie mam czasu odpocząć focuses on not having enough time
• Nie mam kiedy odpocząć focuses on not having any suitable moment/opportunity

In this sentence, nie mam kiedy odpocząć sounds very natural because the second half explains why: the speaker works until six during the week, so there is no real moment available.

Because after mieć kiedy, Polish normally uses an infinitive:

• mam kiedy odpocząć
• nie mam kiedy zadzwonić
• nie mam kiedy zjeść

This works a bit like English to rest, to call, to eat, except Polish does not use a separate word like to before the infinitive.

So:

• odpocząć = to rest
• pracować = to work
• zadzwonić = to call

This is about aspect.

• odpocząć is perfective
• odpoczywać is imperfective

Here, odpocząć is more natural because the idea is I don't even get a chance to rest / have some rest — a single completed act of resting.

If you said Nie mam kiedy odpoczywać, it would sound more like I have no time to be resting / to spend time resting in general, which is possible in some contexts, but less natural here.

So in this sentence, odpocząć is the expected choice.

Here w tygodniu means during the week, usually understood as on weekdays.

So:

• w tygodniu = during the week / on weekdays
• w weekend = at the weekend
• w tym tygodniu = this week

That last one is important:
w tygodniu does not mean this week.

In this sentence, w tygodniu zawsze pracuję do szóstej means something like I always work until six on weekdays.

Polish tells clock time with ordinal numbers, not cardinal numbers.

So:

• szósta = sixth
• jest szósta = it is six o'clock

After the preposition do (until), Polish uses the genitive, so szósta changes to szóstej.

That is why:

• do szóstej = until six o'clock

Not do sześć.

Compare:

• o szóstej = at six
• do szóstej = until six

Yes. The time expression is based on an understood feminine noun: godzina (hour).

So the full idea is something like:

• do szóstej godziny = until the sixth hour

In normal speech, godziny is omitted, but the adjective/number still agrees with it, so it stays feminine:

• szósta (nominative)
• szóstej (genitive/locative form)

That is why times look like this:

• jest szósta = it is six
• o szóstej = at six
• do szóstej = until six

Because Polish usually drops subject pronouns when the verb ending already makes the subject clear.

Here:

• mam = I have
• pracuję = I work

So ja is unnecessary.

If you added it:

• Ja nie mam kiedy odpocząć...

it would sound more emphatic, like I don’t have time to rest... with stress on I.

The version without ja is the neutral, everyday one.

Because bo is a conjunction meaning because, and in Polish it is normally preceded by a comma when it introduces another clause.

So:

• Nie mam kiedy odpocząć, bo w tygodniu zawsze pracuję do szóstej.

This is standard punctuation.

You will see the same with other conjunctions introducing subordinate clauses, for example:

• Zostaję w domu, bo pada deszcz.
• Nie idę, ponieważ jestem zmęczony.

Yes.

• bo = because
• ponieważ = because / since

Both are correct, but the tone is different:

• bo is very common and conversational
• ponieważ is a bit more formal or written

So this sentence could also be:

• Nie mam kiedy odpocząć, ponieważ w tygodniu zawsze pracuję do szóstej.

But bo sounds more natural in everyday speech.

Not completely. Polish word order is flexible, and the sentence can be rearranged for emphasis.

The given order is very natural and neutral:

• bo w tygodniu zawsze pracuję do szóstej

You could also say:

• bo zawsze pracuję w tygodniu do szóstej
• bo pracuję w tygodniu zawsze do szóstej

But these may sound slightly different in emphasis or a little less natural depending on context.

For a learner, the original order is a very good model:

• w tygodniu = sets the time frame
• zawsze = adds the habitual meaning
• pracuję do szóstej = gives the main information