CTC took my ds about 4 hours to complete.

This was after a few weeks of training - it took longer at first.

Wyatt has used HOD from the start, so I imagine up to a month of training would be sufficient otherwise. Here is a thread with each of the suggested time allotments for each of the boxes...
CTC time it takes:
viewtopic.php?f=6&t=3765
If your day is going long, chances are the reasons are some of these...
viewtopic.php?f=6&t=6204&p=45485#p45485
Substituting things that take longer than the time allotments planned for the chosen materials for the guide will naturally make the day go longer. When making your schedule or planning your routine of the day, be sure to add in any extra time for substitutions that take longer. Likewise additions will add time, so that extra time should be added in as well. We thoroughly enjoyed doing CTC as it is written, and our HOD days were consistently 4 hours long (give or take 15-30 minutes).

However, substitutions and additions are certainly possible with CTC, and each family must weigh if the extra time these may incur is worth it. HOD is very flexible, and we loved CTC - such an amazing year of growth for our ds, academically and spiritually!
As far as keeping ds on track with his work, a good schedule/routine helps, as well as a timer. The suggested time allotments always help me make my schedule. Here is my past schedule for CTC, in case anything here may be helpful (Wyatt is my ds who was doing CTC in this schedule)...
It also helped my ds to use the CTC guide as a Student Planner. He had the CTC manual in his hands as often or more often than I did. He checked off his boxes as he completed them. This is very motivating, as dc can see themselves progressing through the plans toward completion.

It also helped to have a 'checkpoint' for us to meet together to go through any independent work he had done. I asked the discussion questions, used the editing tips it the Appendix for helping him correct written work, used the narration tips in the Appendix to help him correct written narration work or for guidance in improving oral narrations, and did any other noted follow-ups at that time. I always read aloud the key idea as we finish boxes, as the key ideas are an excellent summation of the lesson. Those are just a few things that have worked well for us in the upper guides - HTH!

I think you will really enjoy CTC. It was such an incredible study of the ancients - I'd not do it any other way and can't wait to do it again with my other dc soon.
In Christ,
Julie