Mapping out the application’s architecture will generate some questions which are needed to be answered .
What are the nature(s) of the problems/troubles that this application will be dealing with ?
What are the Demographics & Elasticity of Demands ?
What are the Entegration & Legacy issues/situations concerning/demanding of the Application ?
This is very much an OO approach and it WILL require more then 2 hours .
Or you can always go for the " tinned tuna " approach where you know where your aces are at all times and no room for a Joker like the OO .