Women typically get a sense of it right away. It is usually implied and is just a process of confirmation from there. How quickly this impression is established or how it is confirmed will depend on the qualities of the man himself. Obviously, a very good looking, charismatic guy will be perceived relatively quickly as one who is wanted by other women, whereas an average looking, awkward, anti-social guy is much less likely to leave the same impression. But actions also convey/confirm this impression as well. A man who regularly dates women, particularly attractive women, is not going to act thirsty/desperate around every pretty face who gives him the time of day.
It is in how you look and carry yourself. It doesn't need to be brought up and discussed overtly by a man (though it can be at times). It is more often brought up and discussed overtly by a woman who has decided she does not want to share such a man.
When guys tell you to wait this long to do this, wait that long to do that, they are giving you nothing but a very generic, contrived formula on how to create the kind of contrast I spoke of earlier. But this is advice that completely ignores the dynamics between an individual man and a woman. It is advice that focuses solely on curbing the enthusiasm of desperation in the man himself. If you are finding that women perceive you as losing interest in them after 2 days of no contact, then you are already displaying, naturally I might add, a lack of desperation even before those 2 days of no contact, which is why it comes off more like a loss of interest rather than the behavior of a busy man with options.
I'm a very busy man myself, genuinely, so much that I often have to schedule dates 2 weeks in advance sometimes. That is enough for me to get away with texting a woman back within minutes whenever I want to, and as often as I want to. I don't have to follow any timeline rules. Women love the fact that I am responsive even though I am very busy. But If I were always available every single day, the same, quick response time all day every day would likely leave an entirely different impression.