There are no rules. Here are four. Know them, master them, break them.
Length
2 to 6 minutes seems to be about as much time and attention as you can expect from someone sitting at her computer. Pure Pwnage goes way longer and they seem to get an audience. Dr Horrible demanded about 14 minutes at time, but it was only a 3-pack, he's a super-villain and he has Joss behind him. The straight ahead web serials that I like best are all in that 2-6 pocket, not quite a coffee break but a little distraction from the other stuff on your desktop.
The Opening Grab
Yeah, I know. You're only asking for a couple minutes of their time, but still their minds are going to wander and their fingers are going to creep over to that mouse if you don't grab their attention right off the bat. There are a lot of ways to grab an audience: make them laugh, create mystery, blow something up. Grab 'em and do it fast. You have 30 seconds. For f&%* sake, don't waste any of them on an opening title sequence.
Only As Many Characters As You Can Service
The web is no place for a cast of thousands. Two, three… six tops. Five minutes just isn't enough time to get to know many more. A one-hander can work too if done right. Think You Suck at Photoshop.
The Closing Throw
Bring the episode to a conclusion that satisfies your viewers but that also makes them want to come back to watch another. This is a tough one. You have to do both things. The episode has to have that beginning, middle and end shape of a good story. Without an end viewers will be left wondering what the point of it all was. There has to be some pay off for sticking it out and now clicking away at… say, the three minute mark. But your ending can't be so closed that they aren't driven to watch more. You need something akin to a cliffhanger, a little mystery still unsolved, a tease about something great that they must see in the next episode, an event that still lies ahead.
