Over what time of period was slavery legal?

Slavery has been practiced by man since ancient times, for instance it was common in the Roman Empire and is referred to in The Bible.

Since then mankind has come to realise that slavery is abhorrent and gradually it has been made illegal all over the world. The last country to make it illegal was Mauritania in 1981.

While slavery is now illegal everywhere, slavery or practices akin to it continue today in many countries throughout the world.