Ik wacht vijftien minuten.

Dutch often leaves out the equivalent of for when talking about a duration.

So:

• Ik wacht vijftien minuten = I wait / am waiting for fifteen minutes

You do not need a separate preposition before vijftien minuten.

This is very natural in Dutch with expressions of time duration:

• Ik slaap acht uur.
• Hij werkt drie dagen per week.

English often needs for, but Dutch usually does not here.

Because vijftien minuten is a length of time, not the thing or person you are waiting for.

Dutch uses:

• wachten op + person/thing
  
  • Ik wacht op de bus.
  • Zij wacht op haar vriend.

But with a duration, you just say:

• Ik wacht vijftien minuten.

You can also combine both:

• Ik wacht vijftien minuten op de bus.

So op is used for what/who you are waiting for, not for how long.

Dutch main clauses usually follow the V2 rule: the finite verb comes in the second position.

In this sentence:

• Ik = first element
• wacht = finite verb, so it comes second
• vijftien minuten = the rest

That is why the normal order is:

• Ik wacht vijftien minuten.

If you start with the time expression, the verb still stays second:

• Vijftien minuten wacht ik.

That version is grammatically possible, but it sounds marked or unusual in most contexts.

It can mean either, depending on context, because Dutch present tense often covers both simple present and present progressive ideas.

So Ik wacht vijftien minuten could be understood as:

• I am waiting for fifteen minutes
• I wait for fifteen minutes

In real life, context usually makes it clear.

If you want to stress that the action is happening right now, Dutch can also use:

• Ik ben vijftien minuten aan het wachten.

That sounds more explicitly like I am waiting.

A very common Dutch sentence is:

• Ik wacht al vijftien minuten.

The word al is important here. It often gives the sense of already / for ... now, which English often expresses with have been ... for.

So:

• Ik wacht al vijftien minuten. = I have been waiting for fifteen minutes.

Without al, Ik wacht vijftien minuten can sound more like a plain statement of duration, and in some contexts it may even suggest a planned amount of waiting rather than emphasis on time already passed.

Because vijftien is more than one, so Dutch uses the plural:

• één minuut
• twee minuten
• vijftien minuten

This is similar to English:

• one minute
• fifteen minutes

A rough pronunciation guide is:

• vijf sounds somewhat like vayf
• tien sounds like teen
• together: vayf-TEEN

A few useful notes:

• The ij sound in Dutch is not exactly the English ay, but that is a helpful approximation.
• The stress is usually on the second part: vijf-TIEN.

And minuten is roughly:

• mi-NYUU-ten or mi-NOO-ten, depending on accent and how carefully someone is speaking.

No, that is not standard Dutch.

English speakers often want to insert voor because it looks like for, but that does not work here.

Correct:

• Ik wacht vijftien minuten.

Incorrect:

• Ik wacht voor vijftien minuten.

Dutch voor has many uses, but expressing duration after verbs like wachten is generally not one of them.

These are related, but not the same.

• wachten = to wait
• wachten op = to wait for someone/something
• afwachten = to wait and see / wait until something happens

Examples:

• Ik wacht vijftien minuten. = I wait for fifteen minutes.
• Ik wacht op de trein. = I am waiting for the train.
• Ik wacht het af. = I’ll wait and see.

So in your sentence, plain wachten is enough because the sentence is about duration, not about the object of waiting.

That word order is not natural.

If you use the aan het progressive construction, the duration usually comes before it or elsewhere in the sentence:

• Ik ben al vijftien minuten aan het wachten.
• Ik ben vijftien minuten aan het wachten.

The pattern is:

• subject + form of zijn + duration + aan het + infinitive

So:

• Ik ben al vijftien minuten aan het wachten. = I have been waiting for fifteen minutes.

It is grammatically complete.

However, in actual conversation, native speakers often want a bit more context, because waiting usually raises the question for what? or since when?

That is why you may hear more specific versions such as:

• Ik wacht al vijftien minuten.
• Ik wacht vijftien minuten op de bus.
• Ik heb vijftien minuten gewacht.

So the sentence is correct on its own, but depending on context, a Dutch speaker might expect extra information.