What is a good estimate of time a person should spend doing aerobic type excercises without having a great negative effect on muscle gains, and also, is doing aerobic excercise after your workout have any negative effects on muscle gains?

This is a great question. There really isn't any specific research studies that have been done on these subjects. However, there is plenty of empirical data that I have seen and experienced throughout my career so that I can give you a quality response.

The upper limit for duration of aerobic exercise without sacrificing muscle mass is right at 30 minutes. However, most people who are focusing on gaining mass but still want to keep their body fat levels under control will not go more than 20 minutes. Let me say a key point here, however. The intensity of which the aerobic training is done is also of paramount importance. Keep your heart rate at OR BELOW 70% of your age predicted max heart rate (APMHR). You can read more about finding this heart rate if you type in "aerobic guidelines" in the search function on the home page of Fitrex.com.

To answer the second part of your question. If your goal is to gain (or at least maintain) lean body mass than you should do your aerobic exercise at least 4 hours before your weight training session OR any time after. Say you train with weights over your lunch hour or after work, you would be fine doing your aerobic training when you get up in the morning. Because the intensity of your aerobic session is low, doing it after lifting weights will not have any negative consequences concerning hormonal responses (growth hormone, testosterone and cortisol levels) and ultimately, physical recovery. Remember, don't go more than 20-30 minutes and keep your heart rate a the relatively low intensity range of 70% (or below) of your APMHR.