I've wondered how much photosynthesis actually happens on earth, and recently looked up the answer. Current best estimate: 150-175 petagrams per year, or about 0.45 petagrams per day. A petagram is the same thing as a gigaton, but how much is that really?
The most useful comparison I found is that the mass of all living humans is 0.5 petagrams. So every day, plants and algae make enough sugar to rival the combined weight of humanity! Another way to look at it is that a cup of sugar weighs 200 grams. So 0.45 petagrams of sugar is enough to fill up Sydney harbor, or make a sugar cube half a mile on each edge.
Such comparisons aren't entirely fair, because all that sugar isn't just piling up into cubes. Plants use up about half of it as energy. And while the other half gets incorporated into new plant growth, plants are being eaten and converted back into energy just as fast. So really what we are measuring is the daily calorie intake of the planet, which is estimated at ~1% of the sunlight that reaches earth's surface. Hooray for photosynthesis!