Let's say Napoleon had took over Europe. ( First French Empire, with Poland, and Denmark.) He beat Russia, but fail to take over England. How long does this French Empire last?
How beaten is Russia? Did Napoleon's invasion succeed and turn it into a Rheinbund-style puppet state or is it merely obeying Napoleon at the moment because he is powerful and resisting him would be counter-productive?
If it's the former, Napoleon's new order will last much longer than if it's the latter.
Either way, ethnic nationalist rebellions will occur on the Continent sometime, and the United Kingdom will be sending money and weapons to each and every one. The entire rest of Europe will become like Spain in the Peninsular War. Unless Napoleon grants genuine autonomy to his subjects in some kind of Continental federation (a highly unlikely step for a man like Napoleon I) I can't see his empire lasting more than a few decades after his death.