The Tips in detail

 "Each modification of your scene takes longer than estimated earlier."

You simply can't predict how long a specific modification will take since even smallest modifications can give a whole new touch and might lead to completely new scenes. For example »only changing the color of the floor« lead to the "Baum" image as a dawn scene. It was a daylight scene when I started and adding a floor to the "Brille" image lead to the "Diplomat" scene...

Take this into account when trying to estimate the amount of time necesarry to build a scene. Over the time you will get a feeling for certain timespans, ie. how long it takes to build the basic objects or how long you might need to set up a fire, but you will rarely encounter anybody who can tell precisely on the hour how long it takes to get the imagination on the screen.

This is especially important when calculating projects you do for any client. Refer to "How much to charge for a rendering-job?".

As I learned by myself there is a creativity deadline for each artist. This is not really a deadline like the one set by IRS but more some kind of barrier in your mind. Most of my modelsget set up within two or three days. If I do not manage to get the basic setup done within this time, I learned that I never finish them.

As I know from various other designers and artists this seems to me like a common guide: If you can't get at least 80% of your model done within three days^*, it's screwed up and you should better start it again from scratch.

The reason to start after a certain period is subtle. The more you work on any model, the more you tend to bite and lock onto a certain problem. If you can't get the basic problems of design and building done in a flow, the harder it will be to do the whole model. 80% of a model is quite a lot, but the last percent are the hardest. The remaining 20% of any model can take many times the time it took to set up the initial 80%, especially when it comes to animating, interacting models and secondlevel animation.