Learn Hardcore Russian: Я ставлю таймер на двадцать минут, чтобы не сидеть за компьютером слишком долго. - I set a timer for twenty minutes so as not to sit at the computer for too long.

устанавливать таймер – literally “to install / set up a timer”.
Sounds more technical or formal, e.g. in instructions, or when configuring software/hardware.
заводить будильник – “to wind / set an alarm clock”.
Traditionally used with mechanical clocks. You can hear завести будильник, but with таймер it’s much less natural.

So in normal spoken Russian about your phone or kitchen timer, ставить таймер is the default choice.

Why is it ставлю (present tense, imperfective)? How would the sentence change with поставлю?

Ставлю (imperfective, present) is used for:

actions happening now:
Сейчас я ставлю таймер… – “I’m setting a timer now…”
or for regular habits:
Я всегда ставлю таймер… – “I always set a timer…”

Поставлю is perfective future:

Я поставлю таймер на двадцать минут, чтобы не сидеть за компьютером слишком долго.
= “I’ll set a timer for twenty minutes so that I don’t sit at the computer too long.”

So:

ставлю – ongoing / habitual.
поставлю – a single, complete action in the future.

Why do we say на двадцать минут? What does the preposition на mean here?

Here на + accusative expresses intended duration:

на двадцать минут – “for twenty minutes”
на час – “for an hour”
на неделю – “for a week”

It means “for the period of… / with the limit of…”.

Compare:

на двадцать минут – for 20 minutes (the length of the countdown).
через двадцать минут – “in 20 minutes (from now)” – time until something happens.
в течение двадцати минут – “during the course of 20 minutes” – more formal, describes a time span, not usually used with timers in casual speech.

So ставлю таймер на двадцать минут is the natural way to say “set a timer for 20 minutes” in Russian.

Why is it двадцать минут, not двадцать минуты?

This follows the standard Russian rule for numbers:

1: nominative singular – одна минута
2–4: genitive singular – две / три / четыре минуты
5 and up (including 20): genitive plural – пять минут, шесть минут, двадцать минут

So with двадцать, the noun минута must be in the genitive plural: минут.

How does чтобы не сидеть work? Why infinitive, not чтобы я не сидел?

Чтобы introduces a purpose clause (“in order to…”).

When the subject of both verbs is the same (here, “I”), Russian normally uses чтобы + infinitive:

Я ставлю таймер, чтобы не сидеть за компьютером слишком долго.
“I set a timer (in order) not to sit at the computer too long.”

Literally: “I set a timer so as not to sit…”

You could say:

…чтобы я не сидел за компьютером слишком долго.

That’s grammatically correct but heavier, and more typical when the subject of the second verb is different:

Я ставлю таймер, чтобы дети не сидели за компьютером слишком долго.
“I set a timer so (that) the children don’t sit at the computer too long.”

So here the concise, natural form is чтобы не сидеть.

What exactly does сидеть за компьютером mean? Do you literally have to be sitting?

Literally, сидеть = “to sit”, but in modern Russian it often means “to spend time being engaged with” something:

сидеть за компьютером – to be at the computer (working, gaming, browsing, etc.)
сидеть в интернете – to spend time on the internet
сидеть в телефоне – to be on your phone a lot

You don’t have to be physically sitting; the phrase focuses on spending time using the computer, not strictly your body posture.

Why is it за компьютером and what case is компьютером?

За with the instrumental case is used for being “at” something you sit or stand behind to use:

за компьютером – at the computer
за столом – at the table
за пианино – at the piano
за рулём – at the wheel (driving)

So компьютером is the instrumental singular of компьютер.

Other prepositions:

перед компьютером – literally “in front of the computer” (more physical location).
у компьютера – “by / near the computer”.

For the idea “spend (too much) time using the computer”, сидеть за компьютером is the standard collocation.

Could we say перед компьютером instead of за компьютером here?

You can say сидеть перед компьютером, and it’s grammatically correct, but the nuance changes:

сидеть за компьютером – the usual expression for “being at the computer (using it)”.
сидеть перед компьютером – emphasizes the physical position “sitting in front of the computer/screen”.

In your sentence, you want the idiomatic “spend time at the computer”, so за компьютером is the natural choice.

What’s the difference between слишком долго and очень долго?

слишком долго – “too long”, excessive duration, longer than is good or desired.
очень долго – “very long”, just a strong degree, not necessarily bad.

In this sentence the idea is “longer than I should”, so слишком долго is the correct choice.

Is the word order за компьютером слишком долго fixed? Could we say слишком долго за компьютером?

Russian word order is flexible here. You can say:

…чтобы не сидеть за компьютером слишком долго.
…чтобы не сидеть слишком долго за компьютером.

Both are correct and natural. The difference is subtle:

за компьютером слишком долго – slightly more focus on where (at the computer) and then how long.
слишком долго за компьютером – the adverb слишком долго is a bit more prominent, focusing on the excessive duration.

In everyday speech, both are fine; the original order is very typical.

Could we say я ставлю будильник на двадцать минут instead of таймер?

You can say:

Я ставлю будильник на двадцать минут.

It will be understood, but there is a nuance:

будильник – usually an alarm clock that rings at a specific time (7:00, 8:30, etc.), or the alarm function on a phone.
таймер – a countdown timer: you set a duration (20 minutes) and it goes off when that time passes (as on an oven or phone timer).

For a “20‑minute countdown” to limit time at the computer, таймер is the more precise and natural word.