I will assume that your question deals with MYW tickets. The answer is that they will not expire. Disney's one day tickets and their predecessor, general admission tickets, have not carried expiration dates except for ones purchased during December of 2004 when those tickets - and only those one day tickets - expired on January 8, 2005 and were clearly marked as such. If you still had a $3.50 general admission ticket from 1971, you could still exchange it today for a one day ticket.
There are however one day specialty and convention tickets that do indeed carry an expiration date (usually 12/31 of the current year) so be careful what you buy and where you buy it from. Many of these are found on Ebay. These will expire whether used or not.