I see this referenced in many places with NXT for 1440 blocks. How long is that exactly?
1440 is the number of minutes in a day. The Nxt software is designed to generate a new block approximately once per minute. In practice the average time is greater, typically in the 80-90 second range, but can range from a few seconds to hours.
1440 was chosen as the number of confirmations needed before a lease takes effect. This delay reduces the risk of an attack on the blockchain which could result from predicting which node will forge next, much easier to do in a short time frame.
720 was chosen as the maximum number of blocks that can be reorganized on the top of the blockchain. This limit prevents attacks all the way back to the genesis block. Thus 720 confirmations is considered safe even for very large transactions. 10 confirmations is considered safe for small transactions.
The choices of 1440 and 720 in the code design were arbitrary, and just as well could have been 1000 and 500, for example.
1440 blocks in nxt is about 24 hours. Since nxt blocks are generated every minute, thats 1440 minutes.
1440 minutes / 60 minutes = 24 hours
NXT block generation isnt exactly 1 minute as some blocks are solved once every few minutes, but on average its about 1 minute. Hence the 1440 block mentions you see everywhere.