'Year-long' describes something that lasts a whole year, or goes on all through the year. It sits in front of a noun: a year-long course, a year-long study, a year-long argument. Two shades of meaning live in it. Sometimes it means 'twelve months from start to finish' — a year-long trip around the world. Sometimes it means 'it never stopped the whole year' — a year-long drought, a year-long wait for an answer. The second use often carries a feeling: the writer is telling you how long and how tiring it was. Because it is a compound adjective, it takes a hyphen when it comes before a noun, and English speakers usually keep the hyphen even after the verb ('the strike was year-long'), though many writers would just say 'the strike lasted a year' there, which sounds more natural. The same pattern makes month-long, week-long, day-long, hour-long, decade-long — so once you know this one you know all of them.