It really depends on how you manage your inbox, one of the drivers behind us building Tray was the customisation it offers. The triggers that define the rules can cover as many or as few contacts as you wish ie 'anyone I have a meeting with in the next 48 hrs' can be a trigger, or simply based on email content, pre-defined contact groups etc. In most cases this is a handful of rules.

Email rules that are setup within the client or service restrict filtering to contact, folder, content (in some cases). We can react to data that's outside of your inbox, anything from your relationship to the contact based on another web service, to accessing, storing, and sharing with services you use regularly. Beyond this triggers such as time, email load (driven from analytics), location can all come into play. We are really just getting started with the possibilities, a way to interact with custom web apps would be great for developers/companies that use bespoke tools.