I have a copper IUD (mona lisa 5) and a delay in menstruation. When should I be concerned?

Hello, I have had my copper IUD for two months now (December). Before that, I was on the contraceptive pill for almost 10 years with a regular cycle of 4-5 days of periods. Since I had my IUD inserted, I had my first period in January and it followed my cycle on the pill, 28 days after the insertion; it just lasted 2 days longer than usual. Where I am concerned is that I am "late" by 4 days at the moment (I am on my 32nd cycle day since my last period started). I know that for the first three to six months after I have a copper IUD, or even for the first year, my period may fluctuate and be abnormal. But is it normal for the cycle to get longer? At what point should I be concerned about a delay in menstruation with a hormone-free IUD? Thank you very much!