Well, both test E and Deca take about 4 weeks to kick in as you've found.
Guessing what you'll gain is pretty tricky. Everyone is different. Typically people will gain 20 or 25lbs on their 1st cycle if everything is in order (training, diet, rest, etc).
Do you have your Post Cycle Therapy planned out? Along with everything you need? After a 12weeker with Test E and Deca, your definately going to want to make sure your PCT is in good order so that you'll keep those gains.
RIP BigJim33 & GearedUp: You are sorely missed my friends.
Hindsight is always 20/20. But looking back it's still a bit fuzzy.