It's pretty low chance of happening. Not insanely low, but it's not the norm.
Outlaw is designed around this RnG aspect, and from my minimal understanding knowing when to chance a reroll of RtB and when to just keep what you got is part of playing the spec correctly.
Frankly even if your DPS from pull to pull is really RnG it's still super fan to play and has the draw of having stars align and beating pretty much everybody if you happen to get a couple back to back 6 buff procs.