You can get the last reload time of the date for apps from its layout:
You can also get information about when an app was last edited, but that property is not explicitly exposed through the SDK, so you'll have to use the generic "AbstractStructure.Get" method to access it from the app layout. I can recommend the "Qlik Explorer for Developers" tool if you want to investigate what properties are available from different Qlik Sense entities. More information about that tool and instructions on how to download and install can be found here:
As for information about when an app was last opened, I don't think there are any endpoints for doing that, but there might be something in the QRS API that I am not aware of.