Click the little button right next to the big render button on the right to resume the render if you've stopped it. I don't now if this works during lightning but I would assume it does. Whenever you click anywhere on the bryce screen it will stop whatever render you are doing.(yes, we all hate this - a simple warning message would be nice) In figuring out how long it will take, render one frame and look at the time it took, and then multiply it by the number of frames you have in the animation. The numbers may tell you, but I don't know what they mean because I have never gotten lightning to work before. Also try looking in the bottom left of the host computer and you may find info there.