I simply tell them billable time for support on old projects is rounded up to 4 hour blocks - this is with or without a retainer / support agreement. This covers switching time (pulling up virtual machines, getting your notes out etc, finding a Windows XP laptop etc), and scheduling it into current work schedule. I work with the client to ensure we gather work into 4 hour blocks where possible (eg. get them to build a todo list in Jira or equivalent). The idea is to avoid death by 1000 cuts - ie dribs and drabs of 1 hour jobs that they call thru at random.
