Aaron Blake doesn't think it's very high:
So what do the young, the independents, the non-interventionists and the non-Christians have in common? They are all pretty small minorities in the Republican Party. … [A]s [Paul] maxes out among those small, key demographic groups, he’s not able to appeal to others. This appears to be what’s happening in Iowa, and while it might be good enough in a six-candidate field there, it will be tougher to execute it as a successful strategy as the race shrinks.
The Dish 