That being said, it's instructive to look at the Inc. 5000 list of the fastest growing companies in US (http://www.inc.com/inc5000/list/2016/).
The average month-over-month growth for the 100 fastest growing companies on this list varies between 20% and 10%, if my math holds.
Now, here we're talking about companies with revenues of at least $2M/year, that have had a sustainable growth for at least 3 years (those are the Inc. conditions).
Obviously, a startup in its early stages is expected to grow MUCH faster than a 3-year old company, so this gives us a lower boundary on what constitutes an impressive growth. In other words, a successful startup is expected to grow by at least tens of percents each month in its early stages, perhaps even by hundreds, but that feels a bit unrealistic. Given all that I think I'll stick with my initial 50% guesstimate.
Hope that helps.