How long can I keep a low rate on a balance transfer or other introductory rate?
You might be offered an introductory rate on a new card or for a balance transfer. The introductory rate has to stay in effect for at least six months, unless you are more than 60 days late on a payment.
The card issuer is required to tell you
how long you will have the introductory rate and what rate applies after the
introductory period. If the introductory rate is variable, it can still change
during the first six months if the index on which it is based (for example, the
“prime rate”) changes.