Instead of allowing 24 hours, I prefer to agree on a start time with the candidate, schedule the email to go out at that time and expect to receive the reply two hours later. That way everyone has the same amount of time.
Yes, I'd used that method too, but I'm not sure how many developers do their best work when told they have 2 hours max to complete any task. In fact we never put any recommendations on how long the task should take, just that they had a 24-hour window in which to come up with the best submission they could. In some (rare) cases submissions that didn't technically complete all the requirements exactly were considered more favorably than ones that did.