Hi Paulo ,
In tradition you can do it by writing some JavaScript code in preparation of the page using HttpRequestHandler >> RunJavascript action. some thing like below
"setInterval(function(){OsNotifyWidget('"+ ProcessFakeNotify.Id +"', '')}, 3000)"
That would poll the request every 3 second and you can add a FakeNotifyWidget.
You can also refer the URL shard by vikas https://www.outsystems.com/forums/discussion/60214/notify-ui-interface-from-bpt/
That uses same method for polling the request and to notify the screen
And on FakeNotify action of FakeNotifyWidget you can check the task completion of timer by below methods
1. Create your entity and a boolean attribute against the logged in user, set that as a false before the timer run and set as true after successfully completed
2. The Timer systems entities you can get the status of the time
3. You can also check the file existence on the server.
Hope this will help.
Best Regards
Devendra