If you've been banned from Modern Warfare 3 for being a jerk or cheating, you can start playing again in another 5,000 days. If you're a 13-year-old and you catch such a serious ban, you'll be 26.5 years old when you can play again. A serious ban is 5,000 days, or 13.5 years, according to MP1st. But don't worry - these bans are dubbed "temporary" by Infinity Ward. While I doubt MW3 has that kind of shelf-life, it's an interesting and mildly amusing way of presenting what is really a perma-ban.
Infinity Ward Creative Strategist Robert Bowling has confirmed the length of the bans, simply saying that "If you see this message, it means you've been Permabanned (for something bad)."
Indeed. While the counting down of days might give some players some measure of hope, it would probably better if they just sent a message saying "your account has been permanently banned."