I found that the ratio between 1-chains and 5-chains was approximately 12⁴, which would suggest that the ratio between N-chains and N+1-chains is about 12:1.  That would suggest that there are about 12³ 5-chains per 8-chain, or about 1,728:1.

If that ratio holds (there's no guarantee that the ratio of N-chains to N+1-chains is constant) then a miner with 1,728 5-chains/hour should find a block every 10 hours when the difficulty is 8.9, every 20 hours when difficulty is 8.95, or every 100 hours when difficulty is 8.99 (all of these are rough averages).  This math should not be taken as anything other than a crude approximation, but it should give an idea of order of magnitude.  My experience has shown that this approximation is at least within a factor of 10.