If you are working in the garden, please let me know when you're finished.
Let me know when you're finished having lunch with me and I will arrange for your transport
You can not use the second one here.
Take the example of a more practical use:
I am now working on the complex problem of global worming on earth that will soon be released to the media after editing. Any idea or suggestions at this stage are always welcomed.
This is burning topic fit for making more and more people aware of. Let me know when you're finished refining it