If in-game reports suggest that you’ve been boosting, cheating, hacking, exploiting, expressing inappropriate behaviour (racial slurs, abusive language, etc) in Modern Warfare 3 and were banned, you can start playing again in another 5,000 days (13.5 years), which is the serious ban according to MP1st.
Infinity Ward Creative Strategist Robert Bowling has confirmed the length of the bans on Twitter, simply saying the following:
#MW3 Player Banned for 5,000 days?! Yes. If you see this message, it means you’ve been Permabanned (for something bad)