Best thing to do is call customer relations after a reasonable time (I'd say two weeks) and politely enquire if anything is happening 'just wanted to make sure you'd received it'. That tends to give things the kick that they need and I generally have a full written response (admittedly of variable quality) by the next day. When I just leave it I tend to be surprised months later when an email pops up. My record is four months for a response that bore no relation to the email I'd send in.