कितने दिन लगेंगे?

A natural word-by-word breakdown is:

• कितने = how many
• दिन = days
• लगेंगे = will take / will be needed

So the structure is basically:

• how many days + will take?

Hindi often leaves out the English dummy subject it, so where English says How many days will it take?, Hindi simply says कितने दिन लगेंगे?

Because कितना changes form to agree with the noun it describes.

Here, it describes दिन (days), which is masculine plural, so the correct form is:

• masculine singular: कितना
• masculine plural: कितने
• feminine singular/plural: कितनी

Since दिन is treated as masculine plural here, you get:

• कितने दिन

Compare:

• कितना समय लगेगा? = How much time will it take?
  (समय is masculine singular)
• कितने दिन लगेंगे? = How many days will it take?

Because दिन is one of those Hindi nouns whose singular and plural direct forms are often the same.

So:

• एक दिन = one day
• दो दिन = two days
• कितने दिन = how many days

The number or quantity word tells you whether it is singular or plural.

However, in some oblique contexts, you may see a different form such as दिनों, for example:

• कितने दिनों में? = in how many days?

So in your sentence, दिन stays as दिन because it is not in that oblique-with-postposition form.

In this sentence, the verb agrees with दिन.

Since दिन is plural, the future form is also plural:

• एक दिन लगेगा = one day will be needed / it will take one day
• दो दिन लगेंगे = two days will be needed / it will take two days
• कितने दिन लगेंगे? = how many days will it take?

So:

• singular masculine → लगेगा
• plural masculine → लगेंगे

It comes from the verb लगना.

लगना has many meanings in Hindi depending on context, including:

• to feel / seem
• to be attached
• to begin
• to take (time, effort, money)

In this sentence, it has the meaning:

• to take / to require

So कितने दिन लगेंगे? literally means something like:

• How many days will be required?

and naturally in English:

• How many days will it take?

Hindi often leaves out subjects or objects when they are understood from context.

English usually needs a subject:

• How many days will it take?

Hindi does not need an equivalent of that it here. The context supplies it. For example, everyone may already know you are talking about:

• a trip
• a repair
• a document process
• learning something
• shipping something

So कितने दिन लगेंगे? is perfectly complete on its own.

If needed, Hindi can make the topic explicit, for example:

• इसमें कितने दिन लगेंगे? = How many days will this take?
• काम पूरा होने में कितने दिन लगेंगे? = How many days will it take for the work to be completed?

Because दिनों is the oblique plural form, and you use that when a postposition follows it.

In your sentence, there is no postposition after दिन, so the direct form is used:

• कितने दिन लगेंगे? ✔

But when a postposition is added, the noun changes:

• कितने दिनों में काम पूरा होगा? = In how many days will the work be finished?

So:

• no postposition → दिन
• with a postposition like में → दिनों

Yes, and it is very common.

• कितने दिन लगेंगे? = How many days will it take?
• कितना समय लगेगा? = How much time will it take?

The difference is mainly in what you are asking about:

• कितने दिन asks specifically in days
• कितना समय asks more generally about time

So if you want an answer in days, your original sentence is more specific.

It is neutral and widely usable.

By itself, कितने दिन लगेंगे? is a straightforward question. It does not include a pronoun like तुम, आप, or तू, so it avoids a lot of politeness issues.

Whether it feels more polite or more direct depends mostly on context, tone, and what else you add. For example:

• कितने दिन लगेंगे? = neutral
• इसमें कितने दिन लगेंगे? = neutral
• बताइए, कितने दिन लगेंगे? = more polite
• लगभग कितने दिन लगेंगे? = softer, more natural in many situations

A simple learner-friendly pronunciation guide is:

• kit-ne din la-gen-ge?

Roughly:

• कितने = KIT-nay
• दिन = din (like din in English, but with a dental d)
• लगेंगे = luh-GENG-gay

A few useful pronunciation notes:

• त and द in Hindi are usually dental, not the same as typical English t and d
• गे in लगेंगे sounds like gay, not like gee
• The stress is not as strong as in English; keep it fairly even

Usually it really does mean days, but like in English, it can also be used loosely.

Depending on context, it may mean:

• actual calendar days
• working days
• approximate time in days

For example, if someone says:

• रिपोर्ट तैयार होने में कितने दिन लगेंगे?

the answer might mean:

• total days
• business days
• estimated days

So the exact interpretation comes from context, just as How many days will it take? does in English.

You change the time word, and the verb agrees accordingly.

Examples:

• एक दिन लगेगा। = It will take one day.
• कितने हफ्ते लगेंगे? = How many weeks will it take?
• कितने घंटे लगेंगे? = How many hours will it take?

If the noun is masculine plural, you will usually see:

• कितने ... लगेंगे

If the noun is feminine, the forms change:

• कितनी रातें लगेंगी? = How many nights will it take?

So both the question word and the future verb can change to match the noun.