I'm 15 weeks pregnant with my second child. I have not had a ultra sound yet, I don't go back to the dr for 5 weeks. Then he will schedule me a ultra sound so I will be 25 weeks before the ultra sound, I heard the heart beat for the first time yesterday. Does anyone else see a problem with this?

I would never second guess a Physician. Have you asked?

There is no set number of times, nor when. Each doctor or clinic is different.
But according to whattoexpect.com, you should have a level 2 detailed ultrasound midway, and that would be about 19 - 20 weeks.
Before and after that would depend (in addition to doctor preference) on things like spotting, history, medical problems, and your ability to pay or your insurance coverage.