Anything. 24/7. In some cases, it doesn't even matter if it's work-related. I've seen employment contracts in the US which, taken as written, mean that if you work as a programmer and, during an extended vacation, you design and patent a new style of hairbrush, your employer owns that patent.

Now, whether those clauses are actually enforceable depends on what state you live in, but employers do routinely make such broad claims to any and all intellectual property developed by their employees, regardless of what it is or when it's created.