Python 3, 25 bytes
lambda n:len(bin(n**n))-3
If the answer is \$x\$, then \$x+1 > n\log{n} >= x\$\$x+1 > n\log{n} \ge x\$ holds true, which means \$2^{x+1} > n^n >= 2^x\$\$2^{x+1} > n^n \ge 2^x\$. So we can simply count the number of bits in the binary representation of \$n^n\$.