Most of the tourist sites are in downtown Bombay and the journey from the airport is around 1 to 2 hours one way. Bombay traffic is notorious between 6 and 9PM and the Taxis will fleece you for sure and the journey is only for the brave. Do not forget that you need to allow at least 1 hour for inter terminal transfer. I would recommend an airport lounge if you qualify or an airport hotel which are quite expensive after all the taxes. You just have enough time to go to Juhu and have dinner and come back. Leela airport hotel has a nice Indian chinese restaurant and a good Indian one too. They have a bus which picks you up and drops you. Good luck