How many days should I devote to training (in a given week # to include running jumping & throwing ) ?
No less than 5. Preferably 6. Each day needs to include some form of technical training.
How long should I train in a single training session in (hours) ?
This will vary from day to day but should probably range anywhere from just under 2 hours to 3.5 hours a day at this point in your career.
How much time should I spend lifting weights a week and also doing Plyo drills ?
Quite a bit. The decathlon is a speed-power event and this should be reflected in your training.
How many miles should I run in a given week ( given that I will have the 100m, 400m, and 1500m events). ? OR Should I just throw 2 distance runs of 20 – 25 minutes per week into my schedule?
If you HAVE to choose betwuen either I'd chose the latter option. Ideally though I'd suggect not doing more than 1 distance run a week and keeping it under 20 minutes in duration. General fitness through sheer volume of training as well as aerobic fitness through tempo work will be more than adequate for the 400m and 1500m.