How long to spend at the gym and how to get a 6 pack?

Well - there isn't a: specific period of time to spend at the gym. Body fat must be low for abs to show like that. Various exercises yield varying results. I would encourage you to seek out a trainer who has experience working with your age group. Take care.

No set time: In addition to exercise you need to have limited body fat, as you can have that six pack but it can be hidden under adipose tissue (fat) on the body. A good aerobic program along with appropriate diet and strength training will pay off in time. Keep at it and you will see results.

Remember: It's not just how long you spend at the gym. It's also, and more importantly, how you spend the time at the gym. 20 minutes of focused high intensity exercise can be much more beneficial than an hour of very leisure activity at the gym. Aiming for around 30 mins is reasonable, but keep on moving. "6 pack" abs come more from optimizing your diet(and weight) rather than excessive core exercises.