How many times can you present the same material at different conferences before it becomes stale?
It's important also to look at your audience. If you're likely to be giving the same talk to two audiences that overlap significantly, then the time between talks is less important than the fact that the audiences will perceive the talk as 'stale'. 

For example, you might give a talk at a general interest conference, and then a slightly modified version of it at a workshop with specialists, many of whom may not have attended the general interest conference. That would be fine. You might then go on the colloquium circuit and give the talk at venues with only small overlap with the specialist group. 

But if you gave the same (or similar) talk to the same (or similar) audience more than once, that would be a problem.