How to put a countdown in a table using asp.net code?

You don't necessarily need to have a column in your database table for status, you could use an aggregated column when you do queries from the table.
In this way there is no need update the table.

advert_id	name	date_created	description	duration	user_id	cat_id
1	Whatever	2014-10-15	The status will be inactive	P7D	5	8
1	Whatever	2014-10-25	The status will be active	P7D	7	9

Note For duration I am using xsd:duration[^] format and the column type should be VARCHAR(25).

In your query you can aggregate the status using a calculation.
As I mostly work in MySQL I use this syntax, but it should be possible to translate the code to other SQL flavors.

SELECT `name`, IF(DATE_SUB(CURDATE(), INTERVAL TRIM(TRAILING 'D' FROM SUBSTRING(duration, 2)) DAY) > date_created, 'Inactive', 'Active') AS `status` FROM adverts;

(The way to get the number of days out of duration can be improved.)

You can also do this in c#.

DataTable dt = new DataTable();
dt.RowChanged += dt_RowChanged; // Add an event that fires for every new row

dt.Columns.Add("advert_id", typeof(int));
dt.Columns.Add("name", typeof(string));
dt.Columns.Add("date_created", typeof(DateTime));
dt.Columns.Add("duration", typeof(TimeSpan));
dt.Columns.Add("status", typeof(string));

// Here you would use something like
//DbDataAdapter da = new SqlDataAdapter();
//da.SelectCommand = new SqlCommand("SELECT advert_id, name, date_created, duration, '' AS status FROM adverts");
//da.Fill(dt);

// Instead I add the rows manually
dt.Rows.Add(1, "Row1", DateTime.Now.Subtract(new TimeSpan(10, 0, 0, 0, 0)), new TimeSpan(7, 0, 0, 0, 0));
dt.Rows.Add(2, "Row2", DateTime.Now, new TimeSpan(7, 0, 0, 0, 0));

The event handler

static void dt_RowChanged(object sender, DataRowChangeEventArgs e)
{
    if (e.Action == DataRowAction.Add)
    {
        DateTime created = (DateTime)e.Row["date_created"];
        TimeSpan duration = (TimeSpan)e.Row["duration"];
        e.Row["status"] = (created > DateTime.Now.Subtract(duration)) ? "Active" : "Inactive";
    }
}