Ruby
Contact hours depend on your year and college. In Prelims and 1A you have lectures from every caucus, as well as set text lectures every week, which totals to about 8 lectures. You'll also have language classes on top of this (about 3-4 of those, but this is dependent on how much language knowledge you arrive with) and then supervisions, which include weekly essays. As colleges set supervisions, these can vary from 3-6 per week. In 1B and Part II, you'll have lectures for each of your papers, spread out over the terms, as well as classes for your papers, and supervisions. You no longer have language classes in these years. A typical work week is about 40 hours.
Emma
The workload varies greatly from first to third years. As a first year, you can expect to have four language classes a week, and around two lectures a day. In addition, you will have college supervisions, usually around five a week. You should only have to write one essay a week, and the assignments for the other supervisions are all fairly short. The lectures and language classes do not require you to submit any work before hand, although you may be expected to do some readings.
In Part IB, the number of language classes per week drops to two, and the number of supervision drops to around four per week. Lectures can be variable, since many Part IB courses run over four weeks instead of eight, so you are lectured in the same topic twice a week instead of once. Depending of the blend of modules you take, you can end up with a term that is very packed in one section, and very empty in another. However, the lecture load is generally manageable.