Unhide control for a set time

Using a little JavaScript, you should be able to accomplish this.

First give your List Lookup control a Client ID JavaScript variable name:

and add a CSS class to your Label control:

Then, in the Custom JavaScript section under Settings, paste the following code:

NWF$(document).ready(function(){
	NWF$('.pleasewait').css('visibility','hidden');
	NWF$('#' + listlookup).change(function(){
		sleepfor(3000);
	});
});

async function sleepfor(time) {
  NWF$('.pleasewait').css('visibility','visible');
  await sleep(time);
  NWF$('.pleasewait').css('visibility','hidden');
}

function sleep(ms) {
	return new Promise(resolve => setTimeout(resolve, ms));
}

Which I pulled from this Stackoverflow article and modified.

Notice where I use the '.pleasewait' for my label and the '#' + listlookup for my lookup control.

I set sleepfor(3000) which equals 3 seconds. Just change that number to accomodate your required length of time.

End result: