Depending on how well the PlayStation 4 does and how quickly developers move to the PlayStation 4 will all contribute to the decision on how long the console support lasts. Sony currently only has two PlayStation 3 titles coming which are Beyond: Two Souls which is set to arrive next week and Gran Turismo 6. Sony has nothing else on the agenda that they are at least talking about for the PlayStation 3.
No decisions have been made yet, but you can be sure that Sony as will third party publishers will be watching things closely to determine the fate of the future of the Playstation 3.