Software Engineering is an interesting mix of practice and theory. On the theory side, you should make a habit of browsing / googling for things that are a good intersection of interesting to you and novel. Be forewarned, some of the common places to get information (like stackOverflow) can be very intimidating.
The standard expiration date for most checks is 180 days (six months) and personal checks are included in this policy. In the United States, banks are privately owned and not government operated so banks do have the right to refuse your deposit if there happens to be any suspicion of the item's negotiability.
Discovering an unredeemed check might seem like found money until you consider its expiration date. While some U.S. checks state how long you have to cash them on the check itself, others typically expire in a set number of days after being written. Not only does this encourage responsibility on the part of the check's recipient, it protects the issuer from overdrafts due to lack of funds if a recipient fails to redeem the check before it expires.