I was dating a guy once who expected me to meet up with him at a moment's notice (like 30 minutes!) If people do this I usually always say no because I'm either busy doing something else, don't have time to get ready and also don't really like to give the impression that I'm available whenever someone snaps their fingers (and I'm usually not!) However, he would then act hurt and as if I didn't like him.
I think it's always bad form personally unless you have known the person a very long time...