It depends on where you live whether g.c.s are allowed to have an expiration date and how far out that should be. In my state there are no rules governing this, so I could sell a g.c. that expires next week if I wanted.
I usually put a 6 month expiration on g.c.'s to encourage people to come in sooner, rather than later. I used to send reminders close to the expiration date, but that became time consuming. I figure it is up to the recipient to pay attention. I will accept them after the expiration date. From time to time I will go through my records and will send notices to people who have purchased g.c's that have expired and will notify them that the g.c. has not been used and say that I will accept it until ______ date.
I think if your goal is to give your business a little boost and get these people in the door so that they may rebook, then it is not such a bad idea to remind people from time to time.