Time duration to accept / reject offers

[Amby Bhagtani]

I have been making offers for multi-family, and in some cases I have been receiving fairly quick responses but in other cases the sellers go blank. No responses, no counter offer. Is that normal in the industry? I just seems a little odd that you would not even receive a counter. Note: The offers aren't super low ball offers either. Is there a specific time duration in which sellers should respond to offers?