You can use Ext.TaskManager to start and stop arbitrary tasks. See Reference
// Start a simple clock task that updates a div once per second
var task = {
run: function(){
Ext.fly('clock').update(new Date().format('g:i:s A'));
},
interval: 1000 //1 second
}
Ext.TaskManager.start(task);
You can add your conditions to start and stop