Where to split a post? Well first of all, do you have to? Or perhaps the better question is why do you feel that you have to?
Years ago Stephen King was one of the first people to look to the Internet to publish a work, online and in several instalments. That must have been over a decade ago now. I heard about the idea at the time and thought it interesting, but never heard what became of the project. My personal view of Stephen King is that while many of his stories are classics and deservedly popular because of it, his works can be over long and because of that sometimes unfulfilling.
The state of the author’s bank balance however would tend to indicate that hundreds of thousands, no millions of people disagree. LOL. I think the most obvious answer about where to split a post is the simplest, wherever you feel it is necessary to do so.
In the case of blogs that are primarily journals then a day is a day, or maybe a catch-up after a few days absence. OK if you’ve been away for a while and the events of the intervening period are a little too much for your readers to take in at one sitting then perhaps you could split the post or defer the rest until later. But on the whole I think blogs work well as diaries, recording thoughts and events while they’re fresh and clear in ones mind.
When it comes to fictional pieces the question is a little less clear-cut but personally I feel that the worst mistake you can make is to try and stick to a formula. Ideas have a natural span, a lifetime on the page that you as the author will recognise. If you want to leave ‘em wanting more so to speak then create a cliff-hanger by all means, though not at the expense of a story’s flow.
If a piece is too long for someone to read, be that because they are busy, find the subject matter heavy going or for any other reason, that shouldn’t make you consider making the next piece shorter. In chasing the readers you can easily edit a story until it has lost its original power and meaning. At that point you have ceased to write for yourself and your readers …so who are you writing for?