Here is my general guideline that i follow but it will be different depending on your lifting method.
If you are doing 2 sets of 10 reps with perfect form
Stay with the same weight and see how many reps you can do with perfect form. If you can do it 13 or more times with perfect form then up the weight a little bit more
When you move up to the next weight, see how reps you can do with perfect form. If you can do at least 6 reps with perfect form then that is your weight. Stay with that weight until you can do 13 or more reps with it at perfect form.
hope it helped