Difficulty of brute-forcing a seed with missing words? Given a 12 or 24 word seed with one one or more missing words what would be the difficulty of brute-forcing the missing words? E.g. for: 


one missing word
two missing words
three missing words


By difficulty I mean the number of permutations against a dictionary wordlist. 

And the related question: how many words would have to be missing in order to make the attack reliably unfeasible?
Missing 3 words in known positions and there will be 8.577.355.766 permutations.
Based on the speed of an average PC, approximately 10 permutations/second. This would take close to 9927 years(if unlucky and have to try every permutations) Sorry, I forgot we are not dealing with wallet password encryption here.

Anyway, a computer will have to get the private key, calculate a public key and then check it for balance, this process will take time.
Missing 4 words(17.540.692.561.920 permutations) or more, and I don't think anyone will want to try their luck. So maybe 5 words or more and even the government will let it slide.