H1B visa still valid after quitting job

I quit my job in US in Jun 2018 and moved to Europe working for another company there. My I 140 was aprroved in 2016 and my H1B is valid till Dec 2019. Can I still go back and work for the same employer in US if he is ready to hire me back?

How long does it take for the H1B to become invalid after staying out of the country and if my employer has not withdrawn H1B?