Tüketim dönemi otuz gündür.

No. Here, -i is not the accusative ending.

Tüketim dönemi is a very common Turkish structure called an indefinite noun compound. In this pattern:

• the first noun stays plain: tüketim = consumption
• the second noun takes a 3rd-person possessive ending: dönem-i

So tüketim dönemi means consumption period.

Similar examples:

• yaz tatili = summer holiday
• öğrenci kartı = student card
• otobüs durağı = bus stop

So the -i in dönemi is part of the compound, not an object marker.

Because Turkish often does not use a separate present-tense verb to be in the 3rd person.

In English, you need is:

• The consumption period is thirty days.

In Turkish, a noun or noun phrase can act as the predicate by itself:

• Tüketim dönemi otuz gün.

So Turkish does not need a separate word equivalent to English is here.

-dür is an optional copular ending here. It often makes the sentence sound:

• more formal
• more official
• more written
• more definite or assertive

So:

• Tüketim dönemi otuz gün. = neutral, everyday
• Tüketim dönemi otuz gündür. = more formal/instruction-like

That is why -dür is especially common in labels, notices, manuals, and official wording.

Because of vowel harmony.

This ending is often shown as -DIr, where the vowel changes to match the word before it:

• -dır
• -dir
• -dur
• -dür

Since gün has the vowel ü, the matching form is -dür:

• gün + dür = gündür

Also, after a voiceless consonant, the d can become t in similar endings.

Because after numbers, Turkish normally uses the singular noun.

So:

• bir gün = one day
• iki gün = two days
• otuz gün = thirty days

Even though the meaning is plural in English, Turkish keeps the noun singular after a number.

Yes, it can in other contexts.

For example:

• Otuz gündür buradayım. = I have been here for thirty days.

There, otuz gündür expresses duration: for thirty days.

But in your sentence:

• Tüketim dönemi otuz gündür.

it is the predicate of the sentence, so it means:

• is thirty days

The structure of the whole sentence tells you which meaning is intended.

The sentence is arranged like this:

• Tüketim dönemi = subject/topic
• otuz gündür = predicate

This is a normal, neutral Turkish order. Turkish usually places the main predicate at the end of the sentence.

So the pattern is roughly:

• [subject] + [predicate]

Right. Turkish does not normally have direct equivalents of English a/an and the in the same way.

So Turkish simply says:

• Tüketim dönemi otuz gündür.

Whether you translate that as:

• The consumption period is thirty days
• A consumption period is thirty days
• Consumption period is thirty days

depends on context and natural English usage, not on a Turkish article.

Yes, you could, but it is slightly different in form.

• otuz gündür = is thirty days
• otuz günlük = is thirty-day / is thirty days long

So otuz günlük is more adjective-like, while otuz gündür is a noun phrase used as the predicate.

Both can be understandable, but otuz gündür sounds very natural in formal informational wording.

Yes.

Tüketim comes from the verb tüketmek, which means to consume or to use up.

So:

• tüketmek = to consume
• tüketim = consumption

This is useful to notice because Turkish often forms nouns from verbs in this way. Recognizing that relationship can make vocabulary easier to learn.