The above is from my danish-greek dictionary:
Rolf Hesse: Σύγχρονο Ελληνοδανικό Λεξικό (Nygræsk-Dansk Ordbog) ΕΚΔΟΣΕΙΣ ΠΑΤΑΚΗ 2018
The Oxford Greek-English Dictionary also has πολύς for “long”, but it specifies that it is used with words denoting time.
So, one could form sentences like:
“We waited a long time” (“Περιμέναμε πολλή ώρα”)
or
“I won’t stay/remain here a long time” (“Δεν θα απομείνω πολύ χρόνο εδώ”),
“Will I need a long time to learn Greek?” (“Θα χρειαστώ πολύ χρόνο για να μάθω ελληνικά;”), etc.
The main problem seems to be that where Germanic languages (German, Danish, English, etc.) use “long”, others (e.g. Romance) tend to use terms like “much”, e.g., Spanish “mucho tiempo”. Similarly, Greek happens to use "πολύς" ("many", "much") in the examples above. Otherwise, it’s “μακρύς”, “μεγάλος”, etc., depending on the context.