On an empty stomach the effects will become noticeable within 15 min depending on the dose. With higher doses the effects can begin in as little as 7 min. The effects peak at around 1 hr with the experience nearing it's end at around the 3 - 4 hr point. Again with higher doses effects may last 4 - 6 hours. The effects will usually begin with a slight sedation, and a feeling of warmth coming over you body.
Muscular relaxation is also quite noticeable.
The subjective effects are quite hard to describe beyond the word euphoria. The sedation associated with codeine is quite a lot less than that experienced with morphine or other stronger opiates. A strong feeling of contentment is usually also experienced. Most people enter a phase where you become quite content and tend to lose interest in their surroundings. A heavy feeling in the limbs also becomes quite noticeable.