i think as long as it averages over at least 1-2 weeks, thats sufficient to prevent any sort of rampant spamming.
I'm not sure, if "how long can spam last" covers the whole picture. I'd like to ask "how fast can miners deploy new hardware/adjust to an increased cap?" in addition.
mining hardware has little to do with the issue, besides the fact that if large blocks are slow to download it could allow large miners to [unintentionally] start creating small forms while the slower miners create orphans. The mining process is virtually unchanged