Five to ten minutes
Good revision does not need to be very time consuming. If it took you an hour to learn something today, just five minutes' revision tonight will make a difference. Revision can be done in bite-size pieces.

After - 48 hours, one week, one month, three months
Another key to revision is repetition. Studies have shown that people who do no revision may have forgotten as much as 90 per cent of what they learned after six months without follow up revision. Five minutes of revision, within a day after learning something, will mean that in six months you are likely to remember perhaps as much as 40 per cent. Another five minutes or so after the first week, the first month, and then the third month, significantly improves these statistics. 
Through good revision techniques with emphasis on bite-size revision repeated over monthly periods - studies have shown that after six months a person may remember as much as 90 per cent of what they originally learned. And all this comes after only perhaps four five-minute revision sessions.

On the bus, the train, waiting for a client...
Revision doesn't have to be done in a library. With a busy day ahead of you, perhaps you don't have the ten minutes it takes to set yourself up at your desk with your materials so that you can do five minutes' revision. It simply isn't cost/time effective. Instead, why not just pull some of your materials out when you're on the bus or the train? That way, you're not losing any time, and you can easily get your five minutes in. It all counts.