Add a column to your Users table which is a RenewalDate - when the user logs in, check the value against the current date less the "reminder period"
That's almost trivial in C#:
DateTime renewOn = ... get your renew date value from your DB ...
if (renewOn <= DateTime.Now.AddDays(7).Date)
{
... renewal needed
Then all you have to do is show the alert to the user - which again is pretty trivial, but depends on what kind of system you are coding for, and exactly how you want to display it.