You need to have a plan, list exactly what sites you want to visit, ie just the battle sites, but then you would be missing things like fort Eshowe and the graveyard, fort pearson and the graveyard, and the grave of the prince imperial for instance.
Remember there are many isolated gravesites and forts all over Zululand.
Also how long do you want to spend at the sites, do you want to walk the battlefields, Hlobane is easily a full day, Isandlwana and Fugitives Drift can take 3 days for an in depth visit.
What time of year, i have visited in May and October both were not too hot, but you can have rain.
A good estimate is 14 days including 2 days air travel from the UK.