For example, if 200 seconds of images and videos were used in negative mood induction protocol, it is mandatory to have same 200 seconds for positive induction protocol or it can differ +/- 100 seconds or more?.
If the duration of induction differs between conditions, the effects you might find, could be (solely) based on the participants being more tired or exhausted in one of the two conditions. In my opinion you should keep both conditions (or three if you have a "neutral mood" control condition) as similar as possible. That would include first of all keeping the duration of induction the same between conditions. However, you should also keep in mind that the ratio of images vs. videos, (visual) complexity of stimuli etc. could have an impact on how comparable your conditions are.
Thanks Jonas Potthoff and Bartosz Bohaterewicz for your valuable suggestions. Bartosz Bohaterewicz , i have arranged the images in the increasing order of intensity, hence randomising their order becomes trouble, in my study!