Episodic Memory Decoding
The answers to your questions in your post would depend on the scope of your study.  For example, with


  How long should the time between encoding and decoding be?


it would depend on what criteria is set within the scope of the study.  If the people who are asking you to perform the study are asking for specific timescales then you need to stick to them and outline that in your report.

If however you have freedom to do so, as you seem to be asking on a psychological standpoint, it may be beneficial to test different timescales and maybe tabulate the results of memorability against time. You could even test different timescales against different ranges of emotional levels attached to the event.

For example, 

does a person remember a highly emotional episodic memory longer than a non-emotional one? 

What about medium levels of emotions?

You could also add to this with your question on how long snippets should be.

Does a 5sec video snippet help someone to remember better than a single frame image?

If you have the time and freedom to explore, you can conduct a large amount of research on memory.  It would only be down to how wide you can make the scope of the study whilst keeping it within the remit of what you are trying to find out.