Caching Google calendar credentials or service

I have the following code to build a Google calendar service with the Java API (using a service account):

/**
 * Return a google Calendar object for interacting with the calendar API.
 * Return null if it can't be built for any reason
 */
private Calendar buildGoogleCalendarService() throws GeneralSecurityException, IOException {
    String googleUsername = this.getGoogleUsername();
    if (googleUsername == null) {
        return null;
    }
    String path = AuthManager.class.getClassLoader().getResource("").getPath();
    File privateKey = new File(path + "/google_key.p12");
    if (!privateKey.exists()) {
        logger.error("Google private key not found at " + privateKey.getAbsolutePath());
        return null;
    }
    JsonFactory jsonFactory = JacksonFactory.getDefaultInstance();
    HttpTransport httpTransport = GoogleNetHttpTransport.newTrustedTransport();
    GoogleCredential credential = new GoogleCredential.Builder().setTransport(httpTransport)
            .setJsonFactory(jsonFactory).setServiceAccountId(AppProperties.googleAppEmailAddress)
            .setServiceAccountPrivateKeyFromP12File(privateKey)
            .setServiceAccountScopes(Collections.singleton(CalendarScopes.CALENDAR))
            .setServiceAccountUser(googleUsername).build();
    Calendar service = new Calendar.Builder(httpTransport, jsonFactory, credential)
            .setApplicationName(AppProperties.appName).build();
    return service;
}

It works fine with some basic testing, the question is how long will the credentials / service be able to be re-used for? i.e. how many API requests can you make using it before regenerating? This server application may process a high volume of API calls and last for some months between reboots.

Doing some timing, the credential building stage (GoogleCredential credential = new GoogleCredential.Builder()...) takes the most time, approx. a quarter of a second, I'll try caching that to start with and see how it goes but any answers appreciated.