Is a 5 day gap normal? Is it not recommended / too long of a wait?
It's normal for candidates to have scheduling conflicts that mean they might not be immediately available. It's not so normal for candidates to say they're not available when that's not actually the case. If you do this for follow-up interviews as well they might notice the pattern and find it a bit peculiar, but it's unlikely to be a real concern for them.
The reason it's peculiar is that most people wouldn't expect a recent graduate's schedule to be so packed, unless you're currently employed or the volunteer work you mentioned practically counts as a full-time job and its nature is such that you can't skip a few hours or a day at short notice. If that's the case, just explain that when you reply with your availability. People might expect more flexibility for Skype interviews but if your volunteer work prevents you from being online or in a setting where you can conduct a quiet interview then they'll understand.
If you already have plans in those five days, just provide them with the days/hours that have not yet been scheduled. Those are the moments that you're still available after all. If you're fully booked with interviews or pre-existing engagements and won't have time to prepare, just give the first possible day, whether that's four, five or ten days from now. If you are available, there's no need to build in a buffer.
Preparing for an interview shouldn't take more than a few hours at most, they won't expect you to be able to cite their entire company history or identify the C-levels by their photo. And I'm assuming this was a typo but there's also no need to "master the software the company requires" as companies hiring new graduates don't expect them to hit the ground running.
One caveat: I don't know of any companies that would consider a candidate not being available within 5-10 days to be a problem but there could be some out there who are fast-tracking applications or who have pre-planned interview days (government positions come to mind) and not being available could be a deal-breaker there. But those companies generally won't ask you to provide your availability.
Executive summary: don't worry if you're not immediately available but don't build in an unnecessarily long buffer to do unnecessary amounts of prep.