Since my first retreat at Throssel in October 2008, I've sat 30 minutes every day, without fail.
Since my second and 1 week long retreat at Throssel on April 20th 2009, I've sat 2 x 30 minutes every day without fail and 4 x 30 minutes at the weekends.
Up until then I'd sat off and on for 7 years - with long periods of not sitting and longer periods of sitting every day for 15 minutes. Sometimes alone, and sometimes at temples in and around Tokyo.

Why do I have to sit?

The author of the blog goes on to answer his own question.

Read the Zen section.

There are very many blogs talking about meditation. This one comes from a person who has picked up what he learned at Throssel, and uses it. His schedule of sitting is rigorous and that's how it needs to be for some time. Although few actually follow through and do this consistently. One important point though is to remember to purposefully skip a sitting period from time to time. Nobody has to sit, skipping a sitting is a good...sitting period!