A professor wanted me to wait for his decision. It's been about 10 days, what should I write to show that I want to know how long it is going to take?
It sounds like he's asking you to wait because you're his plan B, not his plan A. In other words, he may be afraid that his favorite candidate backs out, and he's keeping you in the wings just in case that happens. 

So should you push the issue? No, not unless you have other offers. And if your other pending applications haven't been responded to yet, it's in your best interest to wait until those other decisions come back to you.