You can specify each time you resume a render, how long it should render for that session. So you can render something in intervals (ha!) and each time specify different time. Or different sampling levels.
I think the flag for time is: -t:60
it's written in the mxcl docs.
I'd still like to be able to change these settings in Maxwell (ie:outside of Max). I know you can do this from the command line but I'm talking about when I add jobs to the queue.