I believe 3 days is enough. A new marketing idea can be implemented in 3 days; a new UI mockup can be done in 3 days. To me, I'll be learning things, but to the founders they can actually benefit from my ideas or executions. It's free after all, and they can definitely take it further after the 3 days (maybe hire me something).
I assume when one says for 3 days, he/she knows that they won't have time after that - or else, they would have agreed for a week/two or so on. What if the idea/mockup was not completed in 3 days, and the trainee was not willing to spend more time...
From a founder's perspective they lost the time. Probably for mitigation they would have someone else implement the idea too... which would indicate that they might not provide the same attention to your implementation.
