What is the optimal workout time?

Many natural bodybuilders would not make good progress, especially in the long term, doing 90 minutes of intense training but it depends upon your training - how heavy, how long rest periods are, what body parts you're training, etc. On the other hand, some guys get good gains from longer, higher-volume training. The only way to find out what works for you is to keep very detailed records so you can compare the results you get from various training strategies. You should probably mix it up for best results anyhow...something like 6-weeks of volume training, followed by 6-weeks of low-volume but heavier training, etc along with lighter weeks and a week or two off every once in a while to recovery and regenerate.