How to properly set session timeout using .netcore 2.0 identity and cookie authentication

Question

I'm trying to use identity with cookie authentication in an asp.netcore 2.0 MVC app. I want to set session timeout to something like 150 days, so users that log in don't need to log in again during a big period of time. I set options.ExpireTimeSpan = TimeSpan.FromDays(150); but sessions are closed after a few minutes, even if the user is actively using the app.

I'm following the official docs and I have this on my Startup.cs:

Within ConfigureServices method:

    services.AddIdentity<User, Role>()
            .AddDefaultTokenProviders();

    // Identity Services
    services.AddTransient<IUserStore<User>, UserService>();
    services.AddTransient<IRoleStore<Role>, RoleService>();

    services.Configure<IdentityOptions>(options =>
    {
        // Password settings
        options.Password.RequireDigit = false;
        options.Password.RequiredLength = 8;
        options.Password.RequireNonAlphanumeric = false;
        options.Password.RequireUppercase = false;
        options.Password.RequireLowercase = false;
        options.Password.RequiredUniqueChars = 3;

        // Lockout settings
        options.Lockout.DefaultLockoutTimeSpan = TimeSpan.FromMinutes(30);
        options.Lockout.MaxFailedAccessAttempts = 10;
        options.Lockout.AllowedForNewUsers = true;

        // User settings
        options.User.RequireUniqueEmail = true;
    });

    services.ConfigureApplicationCookie(options =>
    {
        // Cookie settings
        options.Cookie.HttpOnly = true;                
        options.ExpireTimeSpan = TimeSpan.FromDays(150);
        options.LoginPath = "/User/Login"; 
        options.LogoutPath = "/User/Logout"; 
        options.AccessDeniedPath = "/User/AccessDenied";
        options.SlidingExpiration = true;
    });  

    services.AddMvc(config =>
    {
        var policy = new AuthorizationPolicyBuilder()
                         .RequireAuthenticatedUser()
                         .Build();
        config.Filters.Add(new AuthorizeFilter(policy));
    });

In the Configure method I call app.UseAuthentication();

And to do the login, I do:

var result = await _signInManager.PasswordSignInAsync(email, password, isPersistent: true, lockoutOnFailure: false);

Am I missing any config? I theory I'm setting expiry time to 150 days.

Thanks in advance.

EDIT:* It looks like the problem is the ValidationInterval. I found this issue: https://github.com/aspnet/Identity/issues/1513

I tried setting the ValidationInterval to a large value and now sessions are not closed and users are not logged out unexpectedly. This is the code to set the interval:

services.Configure<SecurityStampValidatorOptions>(options => 
            options.ValidationInterval = TimeSpan.FromDays(150));

I guess I'll have to see how the security stamp validator works and define the validation interval properly, so sessions are not unexpectedly closed but also, a modification in a user's privileges is not ignored due to a stale cookie.