You will always be able to unfuse them. They will not unfuse if you fuse them unless you unfuse them yourself( so there is NOT a time limit)
Just a warning about fusing them, you won’t be able to transfer this to other games because Kyurem-B/W take up 2 Pokemon’s storage space.
Also all levels/evs gained go to Kyurem.
Source: Experience and White Kyurem is cooler than black Kyurem.