when to stop training/rest

I am sure several will post various plans they have followed and or read about as there are several out there.
My opinion is that it all depends on the individual as in age ,fitness level and what you want to achieve.
You have however left out a lot of details such as what was your general fitness level when you started in relation to now? How much do you train, hours a week, miles, intensity, your weight (BMI), calorie intake, resting heart rate, max heart rate and mayeb even your blood levels such as hematocrits.
If you are like me and train in some fashion about 6 days a week and are not exactly youthful any more I would say it is ok to be a bit tired feeling sometimes.
It is also my opinion that there is a balance for each individual and when you are in balance you feel good. The bad part is that life's generalities keep disrupting the balance.

stevechow said:

or an instrument to test my body fitness.

A power meter combined with sound knowledge of using it to its potential is a powerful ally to your own perceptions.

Yoda said:

A power meter, a powerful ally to your own perceptions it is.

Fixed.