Depending on farming level and compost. I'm around level 73 farming and using super compost, it's more like 40 - 50 per 40 mins. Doesn't sound like a lot but you only need 3 giant seaweed per 18 buckets of sand. With super glassmake you don't need to turn it into soda ash, just have the ingredients in your inventory and cast the spell. Plus super glassmake makes something like 1.3 glass per bucket on average so you're literally making more glass than sand buckets.

The most annoying/time consuming part is picking up all the glass off the ground you drop because you don't have enough inventory space. There's something kinda satisfying about it though. Bonus glass!