Hello,
I have let the time-limited Nios II run overnight. There is no problem as long as the processor is connected to the IDE tool, so called "tethered" mode. I no of no other limitation of the Nios II Opencore.
Of course, even with the full edition, there a host of communication problems that I have seen running Nios II that are either self-inflicted or systemic. I seem to get a steady dose of "freezes" from processor paused to verification fails to non-response on the Shell.
Usually I just reset my board and try again.
My two cents.
-Baycool