How To Set Session TimeOut Logout User

Hi All,
Have A Nice Day

when customers log in, I want to put the time in 15-minute counter will automatically log out of client accounts. How to configure issue this?
I have tried over the local configuration in code, but ineffective:

1).Presentation\Nop.Web\web.config
...
<forms name="NOPCOMMERCE.AUTH" loginUrl="~/login" protection="All" timeout="900" path="/" requireSSL="false" slidingExpiration="true" />
...
2). Presentation\Nop.Web.Framework\WebWorkContext.cs
...
protected virtual void SetCustomerCookie(Guid customerGuid)
        {
            if (_httpContext != null && _httpContext.Response != null)
            {
                var cookie = new HttpCookie(CustomerCookieName);
                cookie.HttpOnly = true;
                cookie.Value = customerGuid.ToString();
                if (customerGuid == Guid.Empty)
                {
                    cookie.Expires = DateTime.Now.AddMonths(-1);
                }
                else
                {
                    
        int cookieExpires = 15; //15 minute
                    cookie.Expires = DateTime.Now.AddMinutes(cookieExpires);
                }

                _httpContext.Response.Cookies.Remove(CustomerCookieName);
                _httpContext.Response.Cookies.Add(cookie);
            }
       }
...


Thank alot,