In relation to mortgage loans what constitutes a low credit score?

I have been reading lots about the housing crisis and have learned that the loans for new mortgage loans for people with low scores it getting harder to aquire. What constitutes a low score? 600? 590? 650? 625? I am now ready to buy my first house and am finding out that my scores may prevent me from getting a loan.

In relation to mortgage loans what constitutes a low credit score?

I have been reading lots about the housing crisis and have learned that the loans for new mortgage loans for people with low scores it getting harder to aquire. What constitutes a low score? 600? 590? 650? 625? I am now ready to buy my first house and am finding out that my scores may prevent me from getting a loan.

How should I get a mortgage loan with low credit score?

Ok here is my situation.
I am trying to get the no income verification loan to buy my first house.
I have 10% of the price of the house to pay for down payment.
About my credit: I have everything paid off in full. All debts are paid off in full. but this was recently so my credit score didn’t go up yet it still at 515.

Now my question: Can I get a no income verification loan with my credit score this low?

Also How long does it take to update my credit score?