And, that's not a bad question but usually for reasons other than what the person who is asking it realizes.
There's no doubt that any listing is "hottest" during the first 7-10 days it is exposed to the public. But let me qualify that somewhat. These days many Realtors will place their listings on Zillow as "coming soon" or publicize it through social media, or do a blast to their database and other agents advertising it as a "pocket listing". So does that count? In some ways yes, but if we are just gauging making the listing broadly available and syndicated to all, not selected, websites, then maybe not. And definitely not included in the MLS DOM counter.
What you find in most neighborhoods and price ranges is that homes sold during the first 30 days are very likely to sell at or above the asking price. After 30 days that number goes down and the longer on the market the lower the sales to list price ratio is likely to be.
Here's a few other considerations if you are one of those inquiring how long it has been on the market.
If the property has been on the market for longer than 30 days, has there been a price reduction (or increase). If there was a reduction 3 days ago chances are the seller isn't really looking for a price a lot below what they already reduced it to. But if it hasn't been reduced already, maybe you get lucky and catch it before they do the reduction and avoid competition.
You should also inquire as to whether the listing has been in escrow. If a property falls out of escrow, the DOM counter doesn't start at zero, it just picks up where it left off. That is, unless the listing agent put it in "backup" status.
Let me explain a little but about "backup" status because this is causing a lot of confusion in the marketplace.
Back a few years ago, listing agents would most likely place a property in "pending" status once an offer was accepted. That stops the clock from ticking on DOM. Then it became common practice to put properties in "back-up" status once an offer was accepted. Usually not because the listing agent really is looking for back up offers but most likely they want to receive calls. Major portal websites such as Zillow will continue to display backup status listings along with truly active (no offer accepted) listings because at least in theory the property is available as a back up.
I can tell you from experience that this creates a lot of confusion for the public. At least a few times a week I will bet an inquiry on a property that is not available but is showing up on Zillow, Trulia, Realtor, etc.
So, here's the punch line to all this.
With almost every buyer walking around with a mobile real estate app, they know exactly how long the listing has been on the market, what the seller paid when they bought it and the entire history of the home!