I am departing from London City in a week or two. I'm flying BA, and will have hand baggage only and will have printed my boarding pass. I'm also arriving by taxi. I'm also Oneworld Emerald, in case there's an expedited security line at LCY.
What's the typical maximum I might have to wait at security, and therefore how long before the flight should I arrive? BA unhelpfully only says on their website that check-in closes 20 mins before departure and the LCY website just refers to me to BA.
There's only one security queue at LCY, so flying business or having status doesn't make any difference. There are up to 5 scanning stations, and they do tend to open all of them at busy times. Most people travelling through City Airport are regular travellers, so know how to prepare for security, and consequently things tend to move pretty quickly.
I would say that 20 minutes is cutting it rather fine, especially if you don't know the airport well. You'll need to allow 2-3 minutes from DLR to checkin, then a minute or two to print a boarding pass (longer if checking luggage), then 1-2 minutes minimum from checkin upstairs to security. Even assuming no-one ahead of you, you've then got probably 2 minutes to unpack / security / wait for bags / repack, then walk to the gate.
It's not unheard of for a BA flight to begin boarding before checkin closes, so if you cut it that fine you may struggle to find somewhere nearby in the overhead racks!
If you're after anecdotes, you'll find a lot in these two FlyerTalk BAEC threads on LCY security. Personally, I'd not want to cut it finer than 30 minutes at LCY. There are decent places to eat and drink before and after security, and free wifi throughout the airport, so getting in with a bit more time to spare isn't the end of the world. If you're aiming for 20 minutes and the DLR has a tiny hiccup you're stuffed, so best not risk it!
Worst-case I don't know, typically I would say 20 minutes. Typical worst-case, my guess would be 1.5 hours. When I use LCY it is for one day trips (morning in, evening out), meaning that I am there during rush hours. So I usually worry more about getting to LCY in time.
I try to be at LCY 2 hours before departure. This leaves ample time for delays or missed/wrong connections on the DLR. The downside is that if everything goes well (which it usually does), you end up waiting 2 hours in the departure lounge. Personally I don't mind since in my opinion, the terminal building is one of the most pleasant ones I have seen, small, nice view, internet, electricity, and a good selection of food and drinks.