Gareth,
1) In general, yes. I never regretted recommending an athlete attend a training camp and don’t recall an athlete ever complaining about their overall experience. I’m not an objective observer on this question, however. I’d feel better if other commented on their experiences as athletes.
2) During the offseason, any time. During a formal training plan, best to ask the host how much total volume volume the camp represents, and then adjust your training so that a training week falls on the week of the camp that most closely matches the plan. Don’t find a camp around your plan, rather adjust your plan around the camp by shifting some weeks here and there. I would be skeptical of attending any camp in the final 4 weeks before an event as those are crucial race-specific weeks.
3) I’ve offered as little as 2 days and I’ve done 10 days, both lengths were totally appropriate for the objectives of the respective camp. I don’t have an opinion on how long a camp should be, only the content of the camp within that duration.
4) Similar to my answer #2. Try to match the volume in your plan, but it does not have to be exact. Up to 20% more volume is a reasonable rule of how much you can safely add above your normal/most recent training week.
@Leyla, is this a good time to pre-announce your upcoming camp?
David