hi
maybe you can mimic what linux kernel does when calculating BogoMIPS.
So the idea is, you predict how much iterations it takes during one second. It could be derived using the following shell command:
$grep mips /proc/cpuinfo
bogomips : 3657.19
bogomips : 3657.65
So, roughly it means that in one second, you could do about 3 thousands and 6 hundreds empty loop. Then you could easily create such loop using any syntax you like. BTW, mind you, this is just an approximation, so no guarantee on precision.
PS: watch out for frequency scaling too. It might ruin this BogoMIPS....