The World Cup-winning keeper has been out of favour at Santiago Bernabeu since Los Blancos signed Diego Lopez in January.
Former coach Jose Mourinho preferred Lopez and current boss Carlo Ancelotti has persisted with the former Sevilla stopper, and Casillas admitted that the situation may cause him to consider his options.
"If a player isn't in the action then he starts to think about making a decision," Casillas told reporters.
"I'm human and I'm a footballer. I haven't played for nine months now and it's the same for me. I'm not a robot. Of course I want to stay here and win more titles with Real Madrid. I've been here since I was little, but there will come a day in which I have to take a decision.
"That being said, if in three months things haven't changed and you ask me the same question, well maybe I'll be thinking about leaving."
The 31-year-old has been with Madrid since he was nine.