There is no absolute rule in terms of number of kilograms. What is important is what percentage of your maximal load you use. We can't guess what this maximal load is, because there is beginner and beginner and because it depends on your height and so on, you should just feel it. And of course this maximal load also depends a lot on which exercise you're considering. Trying to suitcase deadlift with 15kg would be useless, but 15kg would be huge for a reverse fly for instance.
To sum up, if you have an adjustable dumbbell going from 0 to 15 kg there's certainly a lot you can do, but if all you have is that one dumbbell with fixed weight it won't be very useful to aim for progressive overload of every muscle, and in the latter case you should rather focus on what you can do in terms of bodyweight exercises.