Which is better: to do low impact exercises for long periods of time or high impact for short periods of time?

Depends...: High intensity workouts can provide massive gains but you have to be careful not to over do it - or risk resulting in an injury that would undo all your progress. 30 minutes twice per week of high intensity training can be very effective. Would consider increasing weight or duration by 10-20% per week to lessen risk of damage. It really boils down to kind of results you want & workout style.