I met an important scientist in my field in a scientific meeting last week. We agreed that I should write a project draft and send it to him, so we could set up a collaboration. Because I want to take as much advantage as possible from this opportunity (I am a PhD student), I want to be very careful with the project draft that I sent him. But this week I have been very busy with other work, and I have not been able to write the draft. My fear is that the interaction will 'cool off' as the days past. It's not been a week yet, and I am planning to write this in at most 2 more days. But in general experience, how long is it ok to wait before sending an email like this?