A fixture between the posts for club and country for more than a decade, Casillas lost his place in the Real team after breaking a bone in his hand midway through last season.
Coach Jose Mourinho, with whom Casillas had a testy relationship and who left to rejoin Chelsea at the end of the campaign, bought Diego Lopez from Sevilla as cover in the January transfer window and stuck with the former Spain international even after Casillas recovered.
Mourinho's successor Carlo Ancelotti has since instituted a policy of playing Lopez in La Liga and Casillas in the Champions League, prompting speculation the latter could seek an exit.
"Players go through moments when if you are not playing a part you think about making some sort of decision," Casillas said at a promotional event in the Spanish capital, according to local media.
"Of course I want to stay here, winning more titles with Real Madrid," he added.
"I have been here since I was a small boy but the day will come when maybe a decision will have to be made.
"I have decided I want to be part of this squad for a long time but if the situation doesn't change in three months and you ask me the same question perhaps I will answer that I am planning to leave."
Spain coach Vicente del Bosque, who also had a stint in charge at Real, has backed Casillas through his troubles at club level.
He selected him for Tuesday's crucial World Cup 2014 qualifier at home to Georgia ahead of Barcelona's Victor Valdes, who has been on sparkling form this season.
Valdes played in Friday's match against Belarus but Del Bosque said that was because Casillas had been sidelined with a minor injury for several weeks.
However, if Casillas fails to command a more regular starting place at Real, even Del Bosque may be forced to bow to the inevitable as Spain prepare to defend their world title in Brazil next year.
"Psychologically, you have to take the positive out of this," Casillas said.
"Ancelotti's decision is the latest challenge, an abnormal situation and I want to win confidence.
"I will fight to get back into the starting 11. I want to play more. To play for your national team you have to be playing for your club."
Real, five points behind La Liga leaders Barca and Atletico Madrid after eight games, host Malaga on Saturday before Italian side Juventus visit in the Champions League on Wednesday.