C# - Creating Web Service Singleton Object

I am creating a singleton object in the first request to the web service and keeping it in the memory. This works fine for first few seconds. If I make a request 5 minutes later, I can see that the singleton object is created again? is my singleton object getting disposed after x no of minutes? how can i make increase the life-time of my object forever ?

public sealed class Singleton
{
static ServerInstance instance = null;
static readonly object padlock = new object();
Singleton() {
}
public static ServerInstance Instance {
get {
lock (padlock) {
if (instance == null) {
instance = new ServerInstance();
}
return instance;
}
}
}
~Singleton()
{
#if DEBUG
ExceptionFactory.Fatal("~Singleton() called!");
#endif
}
}
[WebMethod]
public string OnReceiveEvent(string objectXmlData, string objectType) {
if (!Singleton.Instance.initServerComplete) {
InitServer();
}
return Singleton.Instance.OnReceiveEvent(objectXmlData, objectType);
}
public void InitServer() {
if (!Singleton.Instance.initServerComplete) {
Singleton.Instance.InitServer();
Singleton.Instance.initServerComplete = true;
GC.KeepAlive(Singleton.Instance);
}
}