Is "lang niche sehn" (= lang[e] nich' [g]eseh[e]n) still the most common one? (with that pronunciation?)
(I unfortunately couldn't understand the replies in German above since I am just a beginner in German!)
No, it isn't -- I never heard it as "lange niche sehn

", not even in any dialect I know. May be it exists, but I never heard it.
It is also not used in Saxxony - even if Krümelmonster supposed it in #5.
The standard is "lange nicht gesehen".
There are several coll. variants, influenced by dialect, but most common is in several regions: "lange nicht gesehn"
Sometimes "t" is omitted or almost omitted, "lange nich gesehn".
There are other variants - but they are regional, like "lange nich jesehn" (Berlin, Brandenburg, Potsdam, Magdeburg and some other areas).