These are considered "MLH 2015 Hackathon Season" codes and work till DEC 31, 2017.  So a very long time to use your balance makes these great.  You can only use 1 of this code per account.  If you have other codes from different events some do stack, but I don't promise it due to the wide variety out there.

But if you have not used that event code you should be good to go on using it.  You can only use 1 of these per account they do not stack on each other, but you can create new accounts and have multiple accounts using this code.