3 mins seems reasonable to me - especially if Airbus state that and they obviously know much more about it.
I always winced in the bad old days, when folk would bang on the APU bleed and packs immediately, (and the APU GEN would also be demanding power), the second the APU ready light illuminated.
All I was saying was to err on the gentle side. Metallurgists have no control over the abuse of equipment, but we do, and we might need the APU one dark night halfway across the Atlantic......
Some operators lease all their APUs, so "don't care" about them, but as I say, we pilots might care if it doesn't work when we need it.