I guess it's like any food types... best to try and spread it out through the day but all at once (if the tummy isn't bothered by this) is far better than none at all.
Here's what the NHS has to say on what constitutes 5 a day:
Portion Sizes - Fruit - 5 A DAY

Personally, from experience, i have found that 5 a day is, ideally, the least amount you should aim for unless someone is very young and their digestive systems are not ready for 500 oranges a week!

In 2006 i started juicing and it was a year that i felt (physically) the best i have ever done. I juiced a pineapple, 8 oranges, a kiwi fruit and a handful of grapes every morning which made up about a litre. My skin was never so soft and my eyes were bright and i even felt i slept better at night.

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34yrs old - 6'2 14st 20% body fat [] Target: 15st 11% Body Fat 300Ib bench press and 5 mile run twice a week by Christmas 2008!

Any advise I give is based purely on my own experience or opinion. It should not be solely relied upon as an authoritative statement of the law or as a source of medical advice. Please seek professional advice if in doubt. If this post has helped please click the +rep button.