Perfect Squares between two numbers

Question

Is there any fast way in C (below 1 sec) to find the number of perfect squares between two numbers. For ex. for 1 <-> 10 we have 2 perfect squares 4 and 9. But what about between 1<->2^60 or some other bigger number.

This is slow

while(i*i<=n)
{
    sum+=i==((long long)(sqrt(i*i)));
    i++;
}

where n is lets say 2^60 and we start with i=2.