Sarah at the agency based on this nice little infographic on the best times to tweet. Early afternoon Monday to Thursday being the best for click-through.
After 8pm (after 3pm on Friday) is a no go.
As with everything, make sure you test this out with other days and times. For every piece of research there is another to cancel it out. For example, I know from previous campaigns of some better times to target, but again that could be because of a certain sector we were targeting