This shouldn't be taken as 100% correct, but I seem to remember it being mentioned that each step up in chain length is a 30 fold increase. so an 8 length chain would be 30x more difficult than a 7 length chain, and a 9 length chain would then be 900x more difficult than a 7 length chain.

Again this is from memory, if I can find the post again, I'll add a link (though the math a few posts before this one looks reasonable too!)