Quick answer
Ско́лько лет, ско́лько зим!
skólka lyet, skólka zim — literally “how many summers, how many winters”
The options
The idiom
Literally “how many summers, how many winters” — though лет here is the genitive plural of год (year), which happens to look like “summers”. The phrase is warm, slightly theatrical and completely standard between people who are pleased to see each other.
The neutral version
Давно́ не ви́делись uses the reciprocal ви́деться — to see each other. No друг дру́га needed, because the -ся already carries it. It works in any register, including with colleagues.
What you hear back
Typical mistakes
До́лгое вре́мя не ви́дел.
Давно́ не ви́делись.
A word-for-word translation does not work; the reciprocal verb does.
Ско́лько го́дов, ско́лько зим!
Ско́лько лет, ско́лько зим!
The fixed form uses лет.
Check yourself
What does Ско́лько лет, ско́лько зим! mean literally?