I don't know how trigger happy N. Korea is but you can't hide the evacuation of a major city and it would take hours at least, that's more than enough time for them to see it, order their forces to full alert, and demand you stop the 'provocative act' or be shelled. Once the guys are manning their guns the shells can arrive about a minute after the order is given. I don't know their default level of readiness but in theory a spy in Seoul could call them, then they could call the field commanders to ready up while they decide and issue the attack order mere minutes before the first volley landed. I really doubt they could do it under 10 minutes but delay beyond that would only come from decision making. I gather they have an extensive subway system and wouldn't be surprised if a lot or all buildings had basements with bunkers in mind and I could see getting most people down into those in the allowable time line but I can't see an evacuation of that scale going on fast enough for N. Korea to miss it and not have time to issue threats or attack.