Using Access Codes

I am looking for some help on a topic. I would like to give out access codes to give out free membership to certain people. The problem I foresee with coupons is that people can share the code. I need one time use codes that determine allow a current or new user a certain time length (i.e. 1 month, 3 months, 1 year) of paid membership.

I believe I can easily programmatically set the EOT but I am unsure of other processes that i could cause problems.

Such as:

1) What happens at the EOT with no previous subscription?
2) What happens at the EOT with a previous subscription?
3) What happens if a user with a previous subscription uses an access code and I set the EOT further in the future than the subscription? Will the subscription continue from there?

These are just a few questions I had with this dilemma. The more insight I get on this the better.

Thanks to all in advance for the help.