That depends on the type of workout and the intensity. Within 3 hours if they are training for endurance, 48 hours if training for hypertrophy (getting bigger) or upwards of two weeks if they are training for strength. General rule of thumb is to leave 3 days between body parts for weights, and 24 hours for cardio.

If your workout intensity is good, 30-45 minutes is all you need. No point spending hours at the gym. The intensity of training will determine the rest periods required per set, and hence, play an important role in duration. Tabata training could get over within 30 min, where as an advance weight lifter may take 2 hours due to longer rest time. If you are a beginner, 30-45 minutes is sufficient for you.

Ask yourself, have you been true to your workout?  If that is in place, then look into your rest, recovery and nutrition. If they are in place too, then sometimes the central nervous system is fatigued and requires rest due to over training. Is it true that some people naturally lose weight faster than others?
 Yes, there are many factors but the main two are predisposition to stress, and metabolism. The first one is (in non-nerd speak) the way someone reacts to stress. People will either eat more (comfort eaters) or eat less. In this busy world, stress plays a major role in body composition, and whether a person is a comfort eater or food avoider when stressed will determine their predisposition. Metabolism (in my opinion) is secondary. If someone has a high metabolism (uses more energy to stay alive) then they will naturally lose weight faster than a person with a lower metabolism. Age and gender are the major contributing factors to metabolism; a younger person has a faster metabolism than an older, and a man would have a faster metabolism than a woman. In saying that, of every 100 people who lose weight quickly, 99 will put it back on. Consistency is far more important in a weight loss journey than speed.

The more you train a particular muscle, the lesser chance you give it to grow. Rest is the key to muscle growth. The best option would be to train that particular muscle heavy, 6 to 8 reps if you are an advanced gym goer, and then make sure you are eating to suit the level of intensity you are training at. Get your rest before you train that muscle a week later. One of the reasons to see slow progress could be because of an overlap of training. The Push/Pull routine is best suited to see results.

Feeling pain could be because of muscle soreness from a previous day’s workout. If you have been sedentary for the most part of your life, the muscle would pain because they are in a shock. Bear it and move on, if don’t skip gym days because of it.  If soreness is too much, it’s best you rest or train another muscle group. Take caution if the pain is due to some injury, visit a doctor and rest till you are suggested to.

You can't out train a bad diet.