Not the same situation really, but my sister and i have different fathers, she is 14 years younger than me and my parents (being my mum and step dad) were stressing about how and when to tell her. I decided i would tell her and i think she was about 5. she didnt really get it then, i think she was just too young, she asked a few questions and was happy with the answers and that was it, but i made sure it was talked about from time to time, not big D&Ms, just comments and answered any questions. it was never an issue as we were always really open about it and we never think of each other as half sisters.
IMO its better to do it sooner rather than later, so its no like you have been keeping from her, but you also have to make sure DH is ready to tell her, maybe sit down with him and have a proper chat about it, make him think of the down points of leaving it too long compared with telling her now when she is likely to be more accepting of it.
He might be worried about saying things like ''im not your real daddy'' to her, but there is no need to say that, there are different ways to word it that still gets the facts over without taking anything away from thier relationship.

Im thinking about the relationship i have with my step dad, although i think of him as my dad. i have from about 6 years old, There are different meanings to the word 'father'....in my mind a father is the person who loves you, cares for you, gives you structure in your life and a role model to look up too.....not the DNA which created you. if you can translate that to something an 8 year old can understand it might be a good place to start.