It may take 8 hours to charge a H5 battery with a H4 plug, but your use many vary. It depends on shooting style, and how many shots you're going to take. I've got 3 H4 batteries, and based on experience, I am adding 1-2 of the H5 batteries this year. On average I can do 150-200 shots per H5 battery, but can do more than 250 with the H5 battery. Since I already have 3 batteries, I grabbed a H5 charger so I can have 2 charging at the same time. Toss in my Canon chargers and I've got a full power strip anytime I travel.
If you're going to be in-studio mostly, grab the DC power grip (Part Number: 3043350).