Indeed it is Airbus SOP to use APU Bleed "when the APU is AVAIL" (FCOM PRO-NOR-SOP-04 P 4/12). However, FCTM
PR-NP-SOP-40 P 1/4 states (company with non-customized manuals, my highlight):
"Airbus SOP recommendation is to set the APU Bleed to ON when APU is available. (...) Therefore, to further reduce potential odors in the cabin, the APU Bleed may be selected 3 minutes after APU start. This warm-up time enables the seals to reach their optimum performance and eliminates oil traces in the APU airduct".
While you can use APU bleed straight away (otherwise it would be in the limitations section), my interpretation of this is that it is indeed beneficial to delay its use for 3 minutes after APU start for the reasons mentioned above. Beneficial, but not mandatory or even SOP. We might just call it something like good operational practice, whenever feasible.
I tried to do follow this "practice", particularly on the first APU start of the day, pretty much like someone mentioned previously: start the APU, start the timer when AVAIL lt on, do something else in the meantime, then set bleed on. Mornings are quite cool most of the year at my base, so I'd take further advantage of that and just use one pack all the way until takeoff if the temps in the cabin were comfortable.
Now, something I didn't take into account is the effect of the (short, usually less than 1 hour flt time) flight had on the APU temps for the next flight, as I didn't follow this said practice for subsequent flights. Anyone with any info about this?
Has anyone heard of the APU starting before the inlet door opens?
I think I remember in the really early MSNs you would wait until the "Flap Open" indication on the APU SD page before pushing the start PB. However, after about MSN 25xx or so (in that particular company fleet) you would only need to wait 3 seconds after pushing the APU master PB. APU ECB won't let it start before the flap is open.