How do you answer how long available on an application?

Ask the interviewer questions that help you find out what they are expecting then do something real unique...tell them the truth. The truth may be a small margin for you or a larger one. If there is a lengthy period of time you'd be available tell them and if not tell them. The truth may not work all the time and the truth does not necessarily spare other people's feelings but starting out with the truth with a possible future employer is a good way to start.