I would usually run for up to an hour without taking a drink with me, however if it is hot I would reduce that to half an hour. Hydration is a totally personal matter and it's all about being in tune with your body and what it needs. If in doubt, take some liquid with you.
Also, don't be afraid of doing what I did yesterday when I accidentally overextended my run during a hot lunch hour and had drunk all of my 300ml water. Seeing an inviting pub on the King's Road, I dashed in, sadly not for a refreshing pint, but to ask the kindly barman if he would mind putting some of London's finest tap water into my now empty bottle!