Сто лет не виделись! Рад тебя видеть. Заходи как-нибудь.

Yes — it is a very common idiomatic expression.

Literally, сто лет means a hundred years, so the sentence literally looks like We haven’t seen each other for a hundred years.

In real usage, it usually just means:

• Long time no see!
• It’s been ages!
• I haven’t seen you in forever!

So you should not take сто лет literally here. It is an exaggerated, friendly way to say that a lot of time has passed.

Because виделись comes from видеться, which means to see each other or to meet.

So:

• видеть = to see
• видеться = to see each other / to be seeing each other

That makes не виделись the natural choice for two people who have not met for a long time.

Compare:

• Я тебя не видел = I haven’t seen you
• Мы не виделись = We haven’t seen each other / We haven’t met

In this greeting, Russian prefers the reciprocal idea: we haven’t seen each other.

Russian often leaves out pronouns when they are easy to understand from context.

Here, не виделись already tells you a lot:

• it is past tense
• plural
• reflexive / reciprocal

In context, it clearly means we haven’t seen each other. Adding мы is possible, but it is usually unnecessary:

• Мы сто лет не виделись = also correct
• Сто лет не виделись = more natural and conversational here

Russian drops obvious subjects much more freely than English does.

Тебя is in the accusative case, because it is the direct object of видеть (to see).

So:

• я вижу тебя = I see you
• рад тебя видеть = glad to see you

This pattern is very common in Russian:

• Рад вас видеть = Glad to see you
• Рад познакомиться = Glad to meet you
• Приятно тебя видеть = Nice to see you

So the structure is basically:

• рад
  • object in accusative
    • infinitive

Because Рад тебя видеть is the normal, compact, idiomatic way to say it.

Russian often uses this pattern:

• рад + infinitive

So:

• Рад тебя видеть = Glad to see you
• Рад помочь = Glad to help
• Рад слышать = Glad to hear that

The fuller sentence Я рад, что вижу тебя is grammatically possible, but it sounds less natural in this situation. It feels more explicit and less like a standard greeting.

Also, Russian often omits я when it is obvious, so Рад тебя видеть is perfectly natural.

Yes. Рад is the short form of an adjective, so it agrees with the speaker.

Forms:

• рад = masculine singular
• рада = feminine singular
• рады = plural
• радо = neuter, but this is not used for a person in normal conversation

So:

• a man says: Рад тебя видеть
• a woman says: Рада тебя видеть
• a group says: Рады тебя видеть

This is an important detail, because English glad does not change, but Russian does.

Заходи is the informal singular imperative of заходить.

The verb заходить can mean several things depending on context, but here it means something like:

• drop by
• come over
• stop in

So Заходи как-нибудь means:

• Drop by sometime
• Come over sometime

This is a very common friendly invitation.

Both can be possible, but they are not exactly the same in feel.

• Приходи = come
• Заходи = come by / drop in / stop by

Заходи sounds a bit more like an open, casual invitation to visit. It suggests coming over briefly or informally.

So:

• Приходи как-нибудь = Come sometime
• Заходи как-нибудь = Drop by sometime

In this sentence, заходи feels especially natural because it matches the friendly, conversational tone.

Yes, как-нибудь can mean somehow in some contexts, but here it means sometime or one of these days.

So in this sentence:

• Заходи как-нибудь = Drop by sometime

This use of как-нибудь is very common in invitations and suggestions.

Examples:

• Позвони как-нибудь = Call sometime
• Приходи как-нибудь на чай = Come by for tea sometime

So here it does not mean in some way. It means at some unspecified time in the future.

Because the speaker is addressing one person casually and warmly.

You can see that from:

• тебя = you (informal singular, accusative)
• заходи = informal singular imperative

This suggests the speaker is talking to:

• a friend
• a relative
• a close colleague
• someone of similar age in an informal setting

If you wanted the formal version, you would say:

• Сто лет не виделись! Рад вас видеть. Заходите как-нибудь.

If the speaker is female:

• Сто лет не виделись! Рада вас видеть. Заходите как-нибудь.

Usually yes, but tone matters.

Most often it is a warm, cheerful greeting when meeting someone after a long time.

However, like many expressions, it can be used jokingly or even sarcastically depending on tone and context. For example, if someone disappeared and never contacted you, saying Сто лет не виделись! could carry a playful complaint.

Still, in normal conversation, learners should understand it first as a friendly Long time no see!

Not very well. A word-for-word translation would sound unnatural in English.

A more natural English rendering would be something like:

• Long time no see! Glad to see you. Drop by sometime.

That is better than translating each word literally.

This is a good example of an important rule for learners: with everyday Russian greetings and social phrases, the natural meaning is often more useful than the literal meaning.