Mathematically, you're correct to think that linearly the total distance would be sizable, however don't forget the third axis, depth.
Given the following:
The adult human brain is estimated to contain from 100–500 trillion synapses. (Drachman D (2005). "Do we have brain to spare?".)
Let's go with 150 from your example
... and ...
Every cubic millimeter of cerebral cortex contains roughly a billion of them. (Alonso-Nanclares L, Gonzalez-Soriano J, Rodriguez JR, DeFelipe J (September 2008). "Gender differences in human cortical synaptic density")
Let's calculate the total volume these synapses occupy:
150,000,000,000,000 / 1,000,000,000 = 150,000 cubic mm => 150 cubic cm (
You can picture this as a cuboid 15cm wide by 10cm tall which is roughly the size of a butter bar )
So if all synapses can fit in a 15cm x 10cm cuboid then the clefts could all fit in a significantly less space.
If you want to calculate linear distance for, let's say an average of 30nm clefts then:
150,000,000,000,000,000 * 30nm = 4,500,000,000,000,000nm => 4500km