Hi,
I think that the best way is to create a custom field on the product table.
For example, "availability", for the desired products set the value 24/48h for this custom field in the product edition page.
Then edit the view "product / quantity" and replace the line:
echo JText::_('NO_STOCK').'<br/>';
By:
if(!empty($this->row->availability)){
echo JText::sprintf('AVAILABLE_IN',$this->row->availability).'<br/>';
}else{
echo JText::_('NO_STOCK').'<br/>';
}
And add the following line in the translation file:
AVAILABLE_IN="Available in %s"
By this way you will have the desired result, and you can set different periods of times for different products.