How long is to long?

It's the most important question for everyone training.

Firstly, if you are waking up feeling like crap the next day and are stiff and sore for several days after it's too much. If you didn't know you trained the the day before it's too little. You should know that you trained, when you touch your muscles they should feel slightly tender and you should recover in 24 hours. Sometimes we have to test our mental and spiritual 'mettle' and push farther than we think we can but this should not be done too regularly. Training should build up intelligently over a period of time. IMO 3 times a week is minimum, anything less than that and you are wasting your time. You need to always be skill training but that can cross many borders.

If you train weights, train for connection rather than isolation and if you don't, bodyweight, (yours and others) and/or heavy weapons will do. Some people need more conditioning than others as we are all built (both mentally and physically) differently. Most arts like Tai Chi are designed as a complete work out and you can train differently every day of the week. Variety is the spice of life so constantly change your routines, they should include (in no particular order) qigong, meditation, conditioning, flexibility, form, pairs work, weapons, padwork and sparring.

Train according to your age, condition, nature and objectives. Constantly research, get the best instruction you can and try to do some training every day, make it your lifestyle, the more time you spend training, the more time and energy you will find to do other things. You cannot afford NOT to train, but too many people either train too hard and burn out quickly or never get into it enough to reap the benefits.