My advice is put down whatever amount gets you down to a monthly payment which you can afford with a 15 year mortgage.
30 year mortgages are the devil, you will be working your entire life for the house and pay around $35,000 more in interest over the lifespan of the mortgage if you finance 200,000.
Not to mention that if you do 15, you will be paid off in half the time and you will be netting much more money for the 15 years you'd otherwise still be paying off that 30 year for the remaining 15 years.
All for a measly 200-300 extra a month.
To answer your question more directly, you could say 30% down is a standard but there is nothing wrong with 40, or 50%. Each person is different and you have to see what is best for the situation you're in.