C# async timeout

I have 2 questions but they revolve around the same issue, namely timeout. I have the following piece of code:

public static async Task<string> GetHTMLDataAsync(string url)
    {
        HttpClient web = new HttpClient();
        web.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        string responseBodyAsText = "";

        try
        {
            HttpResponseMessage response = await web.GetAsync(url);
            response.EnsureSuccessStatusCode();
            responseBodyAsText = await response.Content.ReadAsStringAsync();

        }
        catch (Exception e)
        {
            // Error handling
        }

        return responseBodyAsText;
    }

Which seems to work great. It calls the server and fetches resources. But sometime i get some strange null errors when i handle the response, and I am suspecting these to be caused by timeouts. Is there any way to modify the above piece of code to have 30 minutes timeout?

Second question, when doing xaml in wpf i create an Image. I set

Thank you in advance