Čím déle sedím v kanceláři, tím méně času mám na procházku s rodinou.

It means the ..., the ... in English.

So:

• Čím déle sedím v kanceláři, tím méně času mám ...
• The longer I sit in the office, the less time I have ...

This is a very common Czech pattern for showing that one thing changes together with another:

• Čím více pracuji, tím jsem unavenější. = The more I work, the more tired I am.

In your sentence, the two changing things are:

• how long I sit in the office
• how much time I have for a walk with my family

In this pattern, čím and tím are fixed correlative words used with comparatives.

A good way to think of them is:

• čím = the more / the longer / the less ...
• tím = the more / the less ...

They are not being used here with their more literal meanings such as by what or with that. In this sentence, they function as part of a set phrase:

• čím + comparative ..., tím + comparative ...

So even if čím and tím look mysterious at first, in this structure you can learn them as a package.

Because déle is an adverb, while delší is an adjective.

Here, the word describes the verb sedím (I sit), so Czech needs an adverb:

• sedím dlouho = I sit for a long time
• sedím déle = I sit longer

By contrast, delší would describe a noun:

• delší porada = a longer meeting
• delší den = a longer day

So in this sentence:

• Čím déle sedím ... = The longer I sit ...

Because méně normally requires the genitive case.

The noun čas changes to času in the genitive singular:

• čas = nominative
• času = genitive

So:

• méně času = less time
• hodně času = a lot of time
• málo času = little time

This is very common in Czech with quantity words and expressions.

It is locative singular.

The preposition v means in, and when it expresses location, it takes the locative:

• v kanceláři = in the office
• v domě = in the house
• ve škole = at school / in the school

The base form is kancelář, and its locative singular is kanceláři.

Because Czech says mít čas na něco = to have time for something, and after na in this meaning, the noun is in the accusative.

So:

• mám čas na procházku = I have time for a walk

But:

• jsem na procházce = I am on a walk

Here the meaning is not location, but purpose/availability: time for a walk. That is why Czech uses:

• na procházku (accusative)

not

• na procházce (locative)

Because the preposition s meaning with takes the instrumental case.

The noun rodina changes like this:

• rodina = nominative
• rodinou = instrumental

So:

• s rodinou = with the family / with my family

This is a very common pattern:

• s kamarádem = with a friend
• s dětmi = with the children
• s manželkou = with my wife

Because Czech often drops subject pronouns when they are clear from the verb ending.

Here:

• sedím = I sit / I am sitting
• mám = I have

So Czech does not need já unless you want emphasis or contrast.

Neutral version:

• Čím déle sedím v kanceláři, tím méně času mám ...

Emphatic version:

• Čím déle já sedím v kanceláři, tím méně času já mám ...

The version with já is possible, but usually unnecessary and less natural in a neutral sentence.

Not necessarily. In Czech, just like in English, the present tense can describe:

• what is happening now
• a habitual situation
• a general truth

So this sentence can mean something like:

• Whenever I spend longer sitting in the office, I have less time for a walk with my family
• The longer I stay sitting in the office, the less time I have for a walk with my family

It sounds like a general statement about the speaker’s life, not only about one exact moment.

Sedím comes from sedět (to sit), so it emphasizes the idea of physically being seated in the office.

That can suggest:

• being stuck there
• spending time there
• staying at a desk

If the speaker said pracuji v kanceláři instead, the meaning would shift slightly toward working rather than sitting.

So:

• sedím v kanceláři = I sit/am sitting in the office
• pracuji v kanceláři = I work in the office

The sentence as given focuses on the time spent there, almost with a feeling of being tied to the office.

Yes, Czech word order is flexible, but the given version is natural and neutral.

Standard order:

• Čím déle sedím v kanceláři, tím méně času mám na procházku s rodinou.

You could also hear variations like:

• Čím déle sedím v kanceláři, tím méně času na procházku s rodinou mám.

That version puts a bit more emphasis on na procházku s rodinou.

Even with flexible word order, the čím ..., tím ... frame normally stays clear, because it holds the sentence together.

Yes, normally yes.

The sentence has two linked clauses in the čím ..., tím ... structure, and Czech standard punctuation separates them with a comma:

• Čím déle sedím v kanceláři, tím méně času mám na procházku s rodinou.

This matches the pause and the two-part structure of the sentence.

Only partly. A rough literal breakdown is:

• Čím = the / by how much
• déle = longer
• sedím = I sit / am sitting
• v kanceláři = in the office
• tím = the / by that much
• méně = less
• času = of time
• mám = I have
• na procházku = for a walk
• s rodinou = with the family

But the natural English translation is not a word-for-word one. It is:

• The longer I sit in the office, the less time I have for a walk with my family.

That is the best way to understand this kind of Czech sentence: learn the whole čím ..., tím ... pattern rather than translating each word separately.