2-3 feet thick to be effective below 100hz. You can click on the link above and play with the numbers and see for yourself how the thickness impacts the absorption co-efficient as a function of frequency.
It's worth noting the absorption isn't uniform in the 20-200hz range, so there is probably more to it than just stuffing the corners with insulation. You really need to measure your room to see what specific problems you have.
If you don't want to get technical, you can always buy a bunch of insulation rolls and stack them in the corners. If you like what you hear, then figure out a way to make them look pretty, or return them to the store and purchase some finished bass traps from one of vendors here.