If, when you reach the end of the game, you have a maxed party and maxed boxes, the game will progress without you needing to capture Reshiram/Zekrom (previously, the game will not continue until it is captured, and you will be able to capture Reshiram/Zekrom in Black & White respectively in the Dragonspiral Tower. They will once again be at Level 50. Reshiram & Zekrom cannot be captured in their shiny appearances.
This is what serebii has to say for it.
So yeah it will keep re-appearing until it has been caught/you have been defeated/you run out of pokeballs.
If you run out pokeballs/have full boxes and party and it had to be defeated, you can still catch it at the top of the Dragonspiral tower in Icirrus City.
Source Hope I helped!