You should start thinking about increasing your weight when you start to surpass your reps ranges or your lifts become less intense. Basically, when you are able to maintain proper form and the weight becomes easier to move, it is time to increase it. Remember that in order to increase muscle mass, you need to maximize muscle stimulation which is achieved by lifting weights that are challenging to you. In general, you can increase weight all the time as long as your form is correct. Form will change slightly from person to person. Start raising weights in low increments and go up as long as your form is not significantly affected. If you always push to improve, you will.