I think any reasonable mourning period has long passed for Hitler, Stalin, et al. However attacking recently deceased immediately after their death is just not cool. Regardless of what you think of them, assholes have families too. Let them grieve in peace.
How long is long enough? If I remember my history classes correctly, on VE day, Americans held many, many celebrations.
Actually, we could probably pull up some radio news from around that time to see how they covered the story in particular thanks the internet archive.