After April 30th.
Sony will be putting a stop to all DLC purchases for Gran Turismo 5 starting April 20th, so if you want to buy any GT5 DLCs, you should definitely do so while you have the time. Considering the fact that Gran Turismo 5’s online servers will also be shutting down on May 20, I guess it’s fair to say that the game’s active online life has come to an end.
The official GT blog says that, if you want to play any of the DLCs, you should install the ones you’ve purchased from the game’s home menu before April 20th. You can also reinstall previously purchased DLC from the PS Store download list.
Gran Turismo 5 is not the only PS3 game who’s online life is coming to an end. The online servers for MAG, SOCOM Special Forces, SOCOM Confrontation, Resistance: Fall of Man, Resistance 2 and Resistance 3 will all have been shut down very soon.
So yeah, you should download any GT5 DLCs you want to while you still can.