The feature is already visible with some users on the 4.51 version of the Google Photos application. Though not all users currently have access to it. By the sounds of it, it might be making the rounds to users on a slow rollout basis.
As is usually the case with many of the features that Google introduces to its apps. Trashed images in Google Photos having a timer isn't a necessary change, but it is a quality of life one.
Because now you'll be able to look at the trash bin in the app and know how much time you have left before you decide if you want to reverse the deletion on those pics.
The timer on Google Photos trashed images will be above the delete button
If you're on the latest version of the Google Photos app, you can check to see if you have the timer right now.
First, open up the trash bin from the menu on the left side of the app's UI. You know, the same slide out menu that houses the archive, device folders, settings and more.
Once inside the trash bin, tap on any of the images and just above the delete or restore buttons you'll see how much time is left before they're deleted for good.
These should show up for all the images in the trash if the feature is live for you.
Nothing else seems to be new besides the timer
Other than the timer it doesn't look like there are any other new features in the app to speak of.
And there's nothing different about the trash bin other than the timer itself. That being said this is still a change that some users will find to be very useful. Sometimes it's all too easy to blanket delete a bunch of images and not pay attention to what you're getting rid of.
Or maybe you deleted them thinking you were ok with deleting them but now you just want enough time to make up your mind. This new timer will give you that somewhat peace of mind because you'll have a definitive amount of time that you can see.
If you don't see the timer on your own trash bin photos, look for it to pop up sometime throughout the week and beyond. As it can't be too far off.