Depends on the storage, for a file based .raw volume on a standard ext4 it's 16 TiB as that's the maximal file size when 4K blocks are used. For ZFS it's theoretically 16 EiB (~ 16 000 000 TiB) but that's hardly feasible with today's hardware.
So a bit more background for the questions would be required to answer it properly.
There are no direct limits from RBD or QEMU regarding this, but there may be others - e.g., depending on VM disk controller used (IDE, virtIO-scsi, ...) etc. 2 PB is huge but not yet in the "near it's limits" huge, so you may be able to use that as a disk without much need for tuning (just use a filesystem in the guest which can handle this). So, honestly, I'd just try it.