Whenever I've bought a house, I've offered within 24 hours of the viewing.
So if I viewed in the morning, I'll offer in the afternoon. And if I've viewed in the afternoon, then I'll send my offer the next morning.
I've had 9 viewings for my current property, and the agent says that two of them are considering an offer.
That was on Monday!
Personally I think if an offer doesn't materialise within a day or two, then they're just not interested.