I have never owned a bitch before and ours at 8 months has just started to bleed, how long does the bleeding normally last?

The bleeding may only last a feww days or keep going for around 3 weeks.  whatever the lenght of bleeding your bitch can concive at any time up to about 23 days after the start of her season, so should be kept away from any male dogs for at least 3 weeks, and better to be safe up to 4 weeks.

You should not let her off lead at all, and supervise her even in your own garden.

If you cannot be sure of being able to walk her on lead where no off lead dogs will be encountered then she will need to stay on your premises for this time.

If you live in an area where loose dogs are not a problem then walking ehr around the streets on lead avoifing passing dogs should be fine.  If you drive it may be prudent to drive a little way from your home and then walk her, so that you break the scent trail to your home.

I would say she's not safe for 4 weeks, my bitches last season she didn't ovulate until days 20/21 so was still receptive till day 24. 

Like Brainless says, better to be safe than sorry.............4 weeks!