У Вас день за год идет?)))

Hello :) Can you help me explain what this means? I can't seem to get my head round it.

I would take a guess that the meaning is У вас день проходит так медленно, как будто год? ("Is your day going so slowly that it seems like a year?") In other words, when we are very bored, or in great pain, or otherwise in unpleasant circumstances, the time seems to pass more slowly. And I would interpret the use of за + accusative to be similar, in this context, to Она приняла резиновую игрушку за настоящую змею ("She believed that the rubber toy was a real snake.") or Шпион выдавал себя за учителя ("The spy passed himself off as a schoolteacher.")

Is this an expression you heard in a song, or what?

"День за год идет" roughly means "one day equals a year". In what way they equal each other depends on the context.

Expression "X за Y идет" is often used in regards to "стаж" (overall length of service/work, which determins pension size in Russia). In some dangerous professions 1 year of service "translates" into a few years of "стаж", and these people have to work less than others (in actual years) to get a pension.
eg. "Говорят, что у сапёров год за три идет" (i.e. 1 year of service is regarded as 3 "virtual" years).

It can be used in similar fashion in any other situation. For example a driver complains that in his city "год за два идет" for his car, meaning that it wears out faster due to bad roads, a parent can say that while doing a homework with his kid he feels like "час за три идет" either because it wears him out or because time drags too slow.. etc. In computer game it can be a simple matter of difference between virtual and real time.
eg "А на каком сервере ты играешь?,что у тебя год за три идет :D" (c)

In your example it's a joking question (there's a smile after it).

Thanks guys that was really helpful!