Maximising a workout can be different for every individual, as everyone is aiming at achieving different things. Depending on whether you are aiming for cardio or weight training, a workout can differ immensely. 

Personally, I do a mix of both throughout the week and by sticking to a routine and planning out my workout this has helped me greatly. 

I used to think about how long I used to stay at the gym, setting myself time restrictions, and this would eventually make me either rush my workout or make me stay at the gym longer than I would need. I eventually created a workout routine which would roughly take a certain amount of time, and at times I would finish quicker or later depending on the day. This would let me focus on my training rather than how long I should be at the gym.

Making sure you are ready for the gym is also extremely important, if you are tired and not motivated for the gym or your workout at all for that specific day don't bother. This will lead to you not putting 100% for your workout and be detrimental in the long run. This will also make you start to associate the gym as a chore, rather than something that you should enjoy.

Hope this help!

Keep your workout short, around 30-45 minutes! Resistance training is a very effective form of exercise, as it builds strength and endurance. You can do this by using free weights, weight machines, medicine balls, resistance bands and even your own body weight.

Your phone can be an extremely handy, portable workout tool. It can be used to track your heart rate and calculate the distance you run.

For maximum effect, mix strength and cardio exercises into one workout, as this will improve both your metabolism and muscularity.

Keep hydrated and rested and build-up your workouts over time, by adding greater distances or more repetitions for example.