Hi,
Since Virtuemart doesn't support this feature, there is no environment to choose the "time period" for showing the "NEW" badge. It is set to 1 week by default. So; the item that has been added within 1 week, have the "NEW" badge. But you can change this time period to whatever you want.
This is easy though,
Please open this PHP file:
ROOT/template/<yourjoomlatemplatefolder>/html/com_virtuemart/category/default.php
(default.php or flexv2.php, which ever you are using curretcly for category browse page layouts)
Then find this line:
// CHECK IF THE PRODUCT IS NEW OR OLD. IF THE PRODUCT WAS CREATED LESS THAN 1 WEEK AGO, IT WILL HAVE "NEW" BADGE.
$createddate = strtotime(($product->created_on) . " +1 week"); // +1 week can anything you want: +2 week, +1 month, +24 hour so on..
The description is already written there, basically you are going to change this +1 week" to "+1 day" or "+2 week" or even "+52 week" , it is up to you.