I love planning parties, so I do spend a good amount of time searching out the right theme, decorations, goodies, location (if not at home), etc. By 3 months in advance, I'm thinking about the party, and by 2 months, I'm working on real plans. That being said, I try to have invites out 2 weeks in advance, which is generally the norm around here.
I actually thought it rather odd, perhaps even uncouth, to receive an invite a month or more ahead of time. And, it feels rather hasty to get one within the week of the party. I suppose it just feels, in that case, that they haven't really put much effort or thought into it - and perhaps that's exactly what they wanted!

Of course, if I've gotten a heads-up a couple of weeks a head and then the official invite shows up within the week, that's a different story.
I agree that party etiquette is really weird these days. I'm sorry you're having a hard time with your friends.

I was thinking recently that my stash of birthday gifts for friends of my kids isn't really something I need to hang on to, since they have so few friends anymore... We used to go to parties *all* the time because we had a circle of friends amongst whom there were 2-4 kids per family. Having friendships drift apart is hard.

I hope you enjoy your parties, regardless!