What does the song lyric 一等就是一年多 mean?
I googled it and found a forum on Baidu about waiting forever for what looks like a video game to be released. The unhappy camper in the forum writes a lyric verse or two to express his frustration:


  你说过两天来看我
  一等就是一年多
  三百六十五个日子不好过
  你心里根本没有我
  把我的爱情还给我  


Translation (not accounting for poetic style):


  You said you'd come to visit in two days
  But I've been waiting more than a year
  ...


In this context, it means he's already been waiting for more than a year.
This also uses a sentence pattern 
一（V）就（V）, which generally describes something which is done right after another thing, e.g. "我一到家就要睡了" (I'm going to bed as soon as I get home).

In this context, he is using that sentence pattern to stress how long he has waited.