There are 86,400 seconds in a day. So, 1,000,000 divided by 86,400 is equals to 11.57407407407407. That's how many days it would take to count 1 million pennies at a rate of one penny per second non-stop.
A second is the base unit of time. Originally, it was based on the length of the day, but it has since been standardized based on the radiation wavelength of caesium-133.
See Days to Seconds Conversion Table from Calculateme.com.
Of interest, CoinTrackers.com shares a cool list that details the top 25 most valuable pennies ever minted in the United States of America (updated: 2017). These coin values / worth's are not based off common errors like double dies, but rather coins that were issued into circulation as is.
TIP! Find out Calculate Faster Than a Calculator from Wonderhowto.com.