It is difficult to know when to change the oil I just used.

anybody help me?
If you have any good suggestions please tell me!

Thanks!

Different products and its applications decide when to change an oil. For engine lubricating oil the thumb rule is based on changes in its properties.

1. Viscosity : More than 25 % change in Viscosity
2. Water: more than 200 ppm of water
3. TBN : 50% change in TBN
4. Flash Point: 20% drop in Flash point

It may be noted that viscosity change can be corrected by boosting the viscosity with a higher viscosity oil. Similiarly TBN can be boosted by top up with a higher TBN oil a method called "sweetening,"

Water contamination through ingress is too damaging to be corrected easily or on site. Similiarly use of lower flash point too risky for continued use.

In short, lubricating oil contaminated with water and oils with low flash point need to be discarded.

Hussam Adeni

Considering only physical properties for changing the lube oil is insufficient.The other factors like Wear rate analysis,Dirt,Soot, Molecular additive depleation,Oil consumption has to be considered for optimizing oil change interval and for effective condition monitoring.
Dirt ingression,High soot levels elevates wear.
Overstressed conditions depleates molecular additives results in accelerated wear.Over a period of time oil drops its viscosity,increases
oil consumption,leads to excess deposits.

Rather then treating symptoms,treating cause is best maintenance practice.....

Bala.

Condition based Oil analysis is the key.

First know the specs of the fresh oil and then check the deviations with used oil analysis from a reputed laboratary and identify Oil thickening or dilution ,contamination of mositure ,dirt ,internal degradation of oil along with the additive levels and come to a conclusion either usage or discarding the same.

OQA,
Hussam and Bala made a vey good explanation regarding all the oil properties you need to monitor in the oil. However do not forget about the main issue, the machine.
The first rule to determine the best oil change interval is maintain the equipment protection.

Bala made a very good point in saying "Rather than treating the symptoms, treating the cause is the best maintenance practice". It does not make sense to keep treating the same problems over and over again. You tend to waste a lot of time and money in doing that. Fix the problem right the first time.