Unfortunately even discussing this will not change it or alter your frustration (sorry). I always let people know when I am leaving a thread, for the simple fact I believe in treating others as I wish to be treated. So I have never just dropped a thread I have always sent a little note saying my reasons...be it that our styles didn't mesh or I just wasn't feeling it or didn't have time. I am always polite about it and people seem to be accepting ( I have never had anyone abuse me for it).
Sadly some people just get busy and forget to send a note or they don't quite know what to say or don't wish to upset you or have you upset at them so they just avoid and or drop off the radar.
As far as rules go, I am really relaxed about receiving replies (again hoping others will understand that as a busy mum I can't reply every day...and sometimes your just not feeling creative). So I don't tend to worry if a thread is moving slow but after a few weeks I usually send a note asking if they are still interested or to say I have closed the thread because I thought they where no longer interested and if I am wrong to just send me a note when they get back. I find being open and honest is the best bet.
Length of post I like to feel effort returned, I usually write a few paragraphs at least so anything from a couple of paragraphs to a few pages is acceptable to me....If I feel the person isn't interested or is basically using me to get their rocks off (yes has happened) then I usually bow out of the thread.
Being polite honest and up front on what you want and like is the best bet and even discussing where you would like to see the thread go can be helpful.
I know what you mean, as it has happened to me, but it has also happened in reverse where I've dropped the thread for seemingly no reason. So I'll give you my point of view of "the dropper."
Sometimes I join a thread out of boredom, thinking it has some potential there that just needs to be harnessed. Sometimes it just doesn't turn out the way you hope and the interest dies. I've never dropped a thread because I was unhappy with the other person's writing. In fact they've often been much better than me. That could play a part of it, too. Intimidation and fear that I can't keep up. That's mostly in your mind, though. Unless you write like a schoolgirl typing a text message, people here are pretty lenient. Given how you've written your post here, I doubt your writing itself is the problem.
Sometimes real life stresses pop up and even if they don't completely take over the person's life, they may just cause writers block to where they don't reply for a while or even possibly forget about the thread. It's not wrong to politely ask someone who hasn't replied for a while if they are going to anytime soon.
Making a thread with an actual story is a process, and sometimes a difficult one. It's hard to commit to the long haul if it turns out you don't like the character you made or the situation didn't quite turn out as you imagined. If the posts are weeks apart, it can just be the exhaustion of taking so long to get anywhere that kills interest.
In summary, you may have to try different writers and ideas until one really sticks. It sucks that it can take a while but eventually you'll get a really good thread going that satisfies you. If you're simply not that patient (and given the rate of thread death, I would not blame you) then move on.