I've probably been perm banned on OSRS 20 times (and I don't goldfarm!), sometimes botting Zulrah and my bans were always delayed a few days tops. No idea how they deal with RWTers and people who run big farms, wouldn't surprise me if they instaban them like you say.
But I can't see any pros in delay-banning someone for up to a month, only cons. A few days is enough to keep us guessing what got us. Until someone reports back they got banned despite not botting for a month, I don't believe a delay that long exists