Maybe this thread should be in the Legal Forum.
As others have pointed out, "not less frequently than quarterly" means the same thing as "at least quarterly." In other words, the fewest number of assessment payments that a unit owner would make in a year is four, i.e. one per quarter -- it can't be less but it can be more than that.
Making assessments on a condo more than once a quarter would be ridiculous and make a lot of people angry. Once a year would be normal but in unusual circumstances you can do it once a quarter. For instance if you have to do major repairs that were not calculated in the previous years assessment then you could hold a special assessment once every quarter.
Well, around these parts, monthly condo assessments are the norm. I've never heard of a condo association with quarterly assessments, let alone annual assessments, although I suppose a residential homeowners association might levy assessments on a quarterly or annual basis.