This is a two part reply, first to answer your question and then to raise a general licensing issue.
To answer your direct question, the reader extension rights will expire when the credential expires (in your example April 1, 2012). After that time, the user will get a message that depends on the version of Reader.
- If it is Reader 8 or before, they will get a message that the form has changed and that the rights are no longer available.
- For reader 9 and above, the messages were improved, so that it will tell you that your form has gone past the expiration date and that the user needs to get a new form from the creator. It then will differentiate between production licenses and evaluation to tell you which side of things the original RE license fell on. And then it will go on to say that you can still view and print the file (it's still a full PDF) but that you won't be able to save it or perform the other rightes-enabled functions.
But the larger issue is that you appear to not have appropiate licensing in place. Production licenses have long-term licenses (good until 2021 or 2023). The fact that your license expires in 2012 says that you are using evaluation or development licensing and credentials, which all tend to have expirations from three months to two years. You can see details of your licensing in the server UI (do a "get info") or through Acrobat (Niall explained the technique in another thread - see October 24). And if you do either of these, get the credential number (the credential will identify itself as P23 or another P with two digits) and I can decode this for you.
The issue is independent of whether you reader-extend once or you reader-extend as part of a process for each form.