IPA: /ˈjɪrˌlɔŋ/
KK: /jɪrˌlɔŋ/
Continuing for a full year; lasting throughout the entire year.
She enrolled in a yearlong program to improve her skills.
Yearlong → It is formed from "year" (from Old English *geāre*, meaning a period of twelve months) and "long" (from Old English *lang*, meaning having a great extent in time or space). The word "yearlong" means lasting for the entire duration of a year.
Think of a 'year' as a full cycle of twelve months and 'long' meaning it extends throughout that time — that's why yearlong means lasting for a whole year.