Use access_type offline with the Google C# SDK to prevent the need to reauthenticate every hour

Google.cs

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using Google.Apis.Auth.OAuth2;
using Google.Apis.Auth.OAuth2.Flows;
using Google.Apis.Auth.OAuth2.Requests;
using Google.Apis.Util.Store;

namespace YourNameSpace.Google
{
//decompiled with dotpeek and adjusted
    public class OfflineGoogleWebAuthorizationBroker : GoogleWebAuthorizationBroker
    {
        public new static async Task<UserCredential> AuthorizeAsync(ClientSecrets clientSecrets, IEnumerable<string> scopes, string user, CancellationToken taskCancellationToken, IDataStore dataStore = null, ICodeReceiver codeReceiver = null)
        {
            var initializer = new GoogleAuthorizationCodeFlow.Initializer {ClientSecrets = clientSecrets};
            return await AuthorizeAsync(initializer, scopes, user, taskCancellationToken, dataStore, codeReceiver).ConfigureAwait(false);
        }

        public new static async Task<UserCredential> AuthorizeAsync(GoogleAuthorizationCodeFlow.Initializer initializer, IEnumerable<string> scopes, string user, CancellationToken taskCancellationToken, IDataStore dataStore = null, ICodeReceiver codeReceiver = null)
        {
            initializer.Scopes = scopes;
            initializer.DataStore = dataStore ?? new FileDataStore(Folder, false);
            GoogleAuthorizationCodeFlow authorizationCodeFlow = new ForceOfflineGoogleAuthorizationCodeFlow(initializer); //own offline implementation
            codeReceiver = codeReceiver ?? new LocalServerCodeReceiver();
            return await new AuthorizationCodeInstalledApp(authorizationCodeFlow, codeReceiver).AuthorizeAsync(user, taskCancellationToken).ConfigureAwait(false);
        }
    }

    public class ForceOfflineGoogleAuthorizationCodeFlow : GoogleAuthorizationCodeFlow
    {

        public ForceOfflineGoogleAuthorizationCodeFlow(Initializer initializer) : base(initializer) { }

        public override AuthorizationCodeRequestUrl CreateAuthorizationCodeRequest(string redirectUri)
        {
            var ss = new GoogleAuthorizationCodeRequestUrl(new Uri(AuthorizationServerUrl))
            {
                AccessType = "offline",
                ClientId = ClientSecrets.ClientId,
                Scope = string.Join(" ", Scopes),
                RedirectUri = redirectUri
            };
            //ss.ApprovalPrompt = "force";
            return ss;
        }
    };
}

GoogleApiConsumer.cs

using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using Google.Apis.Auth.OAuth2;
using Google.Apis.Gmail.v1;
using Google.Apis.Services;
using Google.Apis.Util.Store;
using YourNameSpace.Google;

namespace YourNameSpace
{
    public class GoogleApiConsumer
    {

        private static readonly string[] Scopes = { GmailService.Scope.GmailReadonly };
        private const string ApplicationName = "ApplicationName";

        private readonly string _userId;
        private readonly string _secretsPath;
        private readonly string _tokenFolder;

        public GoogleApiConsumer(string userId, string secretsPath, string tokenFolder)
        {
            _userId = userId;
            _secretsPath = secretsPath;
            _tokenFolder = tokenFolder;
        }

        public void GetEmails()
        {
            UserCredential credential;
            using (var stream =
                new FileStream(_secretsPath, FileMode.Open, FileAccess.Read))
            {
                var secrets = GoogleClientSecrets.Load(stream).Secrets;
                var store = new FileDataStore(_tokenFolder, true);

                //use your own offline implementation instead of Googles default
                credential = OfflineGoogleWebAuthorizationBroker.AuthorizeAsync(
                    secrets,
                    Scopes,
                    _userId,
                    CancellationToken.None,
                    store).Result;
            }

            var googleClient = new BaseClientService.Initializer()
            {
                HttpClientInitializer = credential,
                ApplicationName = ApplicationName
            };
            // Create Gmail API service.
            var service = new GmailService(googleClient);            
            
            var request = service.Users.Messages.List(_userId);
            request.IncludeSpamTrash = true;
            var response = request.Execute();
            var message = response.Messages.First();
            // use emails
        }
    }
}