How much time, on average would it take to get a block with 20kh/s?
You can get 1800 DOGE per day with 20Kh/s http://dogecoinfaucet.com/dogecoin-mining-calculator/ Cannot tell you the exact time to find a block, but it will take far too long ! and on a pool someone else would probably find it before you. Hope you aren't thinking of solo mining.