No seriously 1/2" uses about 1/2" per joint and 3/4" uses 3/4" per joint.
This isn't a rule! It's not set in stone and I will tell you that I have never in my life measured solder.
You are right If you don't pay attention when soldering the top joint a lot of solder will be dripping out of the bottom joint.
More important than how much you use is cleaning the copper good and fluxing good and not overheating the joint.