Seller not responding to offer

[Leibel C.]

So after a little marketing, and being contacted directly by sellers, I made two unofficial (starting point) offers last week via email.
So far, neither seller has even acknowledged receiving the email, let alone saying "yes" or "no".

How long before I can assume the offer wasn't taking seriously, and that they have no intention of responding? Should I expect an immediate response, or does it sometimes take several days?

There are no brokers in between our negotiation.

Thanks