There is no guarantee you will mint at the 20 day mark. 20 days is the earliest possible, but more than likely it will take several days longer than 20 days before you mint coins. It is comparable to PoW mining in that if you have a small amount of hashpower, it will take you longer to find a PoW block; the same holds true in PoS minting in that a small amount of coins will take you longer to find a PoS block and mint coins. If you have like 1 mintcoin, your probably won't generate ANY new coins, because the probability is so small, just like if you tried to use your cpu to find a PoW block in Bitcoin right now. If you want to mint closer tot he 20 day mark, then it helps if you acquire more coins, as each coin has power to accrue coin age and just like mining gear generates hashpower.