In message OYtco.163011$6C1.65588@hurricane, at 13:38:07 on Mon, 23
Aug 2010, Robin remarked:
you should easily
manage plane seat to exit in 20 minutes, particularly if not checking
luggage)

I've not used London City recently (and am a UK national). The OP
appeared to be posting from the USA and I don't think has given
nationalities. So possibly a bit more than 20 minutes if arriving from
outside the UK and hits a queue for immigration?

But they only use small planes (and the bigger ones like the BA A318
service has only a very small number of passengers). Which only arrive
one at a time - looking at what's happening tonight:

8.20 Edinburgh BA - Embraer 190
8.40 Geneva Swiss/BMI - Avro RJ100
9.00 Zurich Swiss/BMI - Avro RJ100 (last flight).

--
Roland Perry