I have a strained hamstring. should i be stretching while allowing it to heal and how long before i can start running again?

Yes: Hamstring injuries are very common. They also can be very problematic. While you are recovering stretching is essential however you're not stretching hard. You're stretching to release tension and to give normal length to your hamstring muscles. Gentle high rep/low weight work out exercises are also recommend. The key is to exercise to induce healing without re-creating your injury. Good luck.