Like Recharge module

Hi all,Advance Thanks

I have Build Windows application using c#.this is like recharge  module.  my below code its working fine. i have give $ 600 this is validity up to 6 months.

My Query is: but it will only consume 60 hours till 6 months.how can i solve this. alert will come every consuming hours.

int Rs = 0;
                Rs = Convert.ToInt32(txt_AmountPayperUse.Text.Trim());
                if (Rs == 600)
                {
                    DateTime today = DateTime.Now;
                    txt_FromDatePayPerUse.Text = today.ToString();
                    txt_TillDatePayPerUse.Text = today.AddMonths(6).ToString();
                    MessageBox.Show("This Pass Valid Upto 6 months");
                }
                else
                {
                    MessageBox.Show("Enter Amount Only 600");
                }