I sent over 5 eggs to hatch, so I had a full team of 6 from the start, and I spent a while levelling them up to 10 before I went into Trainer School. So I guess someone who had a smaller team or who might not have done that grinding might have been able to get through it faster, but also they might have had a harder time and taken longer to do things so it might balance out.
Either way, without rushing through it, I feel like 6 hours for the first island sounds about right.
What was your plan for your remaining 2 party members?
(Btw be careful if you quote my post again, so you don't accidentally see what's in the spoiler in plain text. Unless you don't mind the spoilers lol)