The previous site logged a user out if they were inactive (no page loads) for 5 minutes. At least it intended to as this didn’t always work. The new site launched with the same 5 minute session expiry but after users indicated they were being logged out too often this was raised to 15 minutes.
Check the “remember me” box when logging in and your session will be refreshed as soon as you visit the site again. Just make sure not to do so on shared or public computers so that others can’t access your account.
Note: the users online totals will be higher because of this change, as users are counted as being online for 15 minutes after their last page load instead of 5.