Writing Something

 
public class SystemUser
{
    public SystemUser()
    {
        FirstReqTime = DateTime.Now;
        ReqCount = 1;
    }
 
    public DateTime FirstReqTime { get; set; }
    public int ReqCount { get; set; }
 
    private bool captcha;
    public bool RequiersCapthca
    {
        get
        {
            if (ReqCount > 5)
            {
                TimeSpan ts = DateTime.Now - FirstReqTime;
                if (ts.TotalSeconds > 5)
                {
                    captcha = true;
                }
                else
                {
                    captcha = false;
                }
            }
            return captcha;
        }
        set { captcha = value; }
    }
}
 
 
public class BasePage : Page
{
    public BasePage()
    {
        //
        // TODO: Add constructor logic here
        //
    }
 
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
 
        SystemUser scu = Session["CaptchaControl"] as SystemUser;
 
        if (scu == null)
        {
            scu = new SystemUser();
            Session.Add("CaptchaControl", scu);
        }
        else
        {
            scu.ReqCount++;
        }
 
        if (scu.RequiersCapthca)
        {
            Session.Add("ShowCaptcha", true);
        }
    }
}