Depends on the size of it.

You can estimate 30 mins per 500g at 180deg. Personally I like to cook for a little longer at a slower temp.
If you rub oil and salt into the roast and also roast it over a pan of water it will be nice and tender. Also take it out of the oven 1/2 hour before you are going to serve it and let it rest under foil before you slice it.
It also is good to let it be a bit pink in the middle because then it won't dry out.

MMMMM I've got one in the freezer ATM and I think it's in danger of being dinner tonight!