We arrived about 8 minutes early and were told we needed to wait a few minutes. It had to be 5 minutes.
I have been let in 5 minutes early, but I think the 15 minutes after depends on the CM.
It doesn't. The only exception is shows that only have the short window. For rides that have an hour window, it's not the kind of thing CMs decide- your band will automatically turn green whether you scan it 5, 10, or 15 minutes after your window passes. I worked at the fastpass entry points of my attractions for months and dealt with this situation many times.
Other CM posters have reported that the built-in system grace periods can be overridden by them depending on the attraction and circumstances. For this reason, I advise against relying on the GP. I do not want someone to plan on showing up late and risk being denied entry, even in rare cases.
Rumor has it the grace period is 15 minutes, but that is never a guarantee. I've tested the 5 minutes early grace period, but never the 15 minutes late.
So it seems that IN THE SYSTEM -- when Mickey will turn green -- there is a built-in 5 minute early and 15 minute late grace period.
Whether it extends beyond that would be up to the discretion of the CM. I would think extending BEFORE the window would be exceedingly rare, and AFTER would depend on extenuating circumstances, the guest's attitude, crowd levels, what kind of day the CM was having, etc.
I would think it would be tough for a CM to "take away" the built-in grace period (e.g., not allow someone though the FP line 3 minutes early, or 7 minutes late), because the touchpoint will be green, after all.