The claims made by the manufacturers for writing to BD-RE disks vary tremendously, from 1000 times (Verbatim) to an over the top estimate by TDK of 10,000 times. I doubt any of us in the forums can give you a definitive answer.
I haven't used 50gb BD-RE, only 25gb, but the very first format of the disk has to be a full format, and takes quite a while. I haven't timed it, but I just go do something else for an hour or so. Burning the data takes just as long at 2x. Once you have done the full format, you can just do quick erases after that, which take very little time at all. I use ImgBurn for all this.
Perhaps someone with direct experience using 50gb disks will chime in.