Starting with version 188.8.131.52.1, Exadata Cells use Pro-Active Disk Quarantine to override any setting of DISK_REPAIR_TIME. This and some other topics related to ASM mirroring on Exadata Storage Servers is explained in a recent posting of my dear colleage Joel Goodman. Even if you are familiar with ASM on non-Exadata Environments, you may not have used ASM redundancy yet and therefore benefit from his explanations about it.
Addendum: Maybe the headline is a little misleading as I just got aware. DISK_REPAIR_TIME set on an ASM Diskgroup that is built upon Exadata Storage Cells is still in use and valid. It is just not referring to the Disk level (Griddisk on Exadata) but instead on the Cell level.
In other words: If a physical disk inside a Cell gets damaged, the Griddisks built upon this damaged disk get dropped from the ASM Diskgroups immediately without waiting for DISK_REPAIR_TIME, due to Pro-Active Disk Quarantine. But if a whole Cell goes offline (Reboot of that Storage Server, for example), the dependant ASM disks get not dropped from the respective Diskgroups for the duration of DISK_REPAIR_TIME.