Or if you know some basic performance characteristics of your machine (i.e. memory bandwidth + processor speed + number of cores) and roughly how much work you

Or if you know some basic performance characteristics of your machine (i.e. memory bandwidth + processor speed + number of cores) and roughly how much work you need to do (i.e. number cycles to do X), some back of the envelope calculations can often give you the same insight (i.e. "is X feasible with compute resource Y?").