$\Theta(\sqrt{N})$.
We know that each iteration of the for loop takes constant time, so we need to figure out how many iterations of the for loop occur; let this mount be $k$. At every step, $i$ increments by 1 so we are essentially accumulating in $s=1+2+3+\cdots+k$ until $s>N$.
Therefore, we want to solve for $1+2+3+\cdots+k=N$ which approximately simplifies to $k^2=N$, which we solve to obtain $k=\sqrt{N}$.
Our runtime is determined by $k$ which is therefore $\Theta(\sqrt{N})$.