Exempt employees aren't required to work any set number of hours per week, but you can generally reduce their pay by 20% (in a 5-day workweek) for every day they don't report to the worksite.
On the other hand, if the workload requires them to work more than 40 hours in the week, you're not required to pay them overtime.
Be careful asking exempt employees to use a time clock - for one, it's just flat-out insulting, and for another, a clever lawyer could claim (because of your action) that their jobs are actually non-exempt because you're treating them like hourly employees.

