I never understood this but I have seen it places on different sides of the table. How is it determined who places it where and why? I know this is a silly question but it is something I have observed whilst watching people play chess on Youtube.
I think it's black that define where the clock will be placed. In some of the tournament, it's a rule, but I think that it's not international or general rule. For me as left-handed, when black place the clock, I reach across the board.
90% of the time, it is placed on the right side as most players are right handed but the one who controls black chooses. This is the care for standard rules and regulations for tournaments but in casual play, most people don't mind who does or some don't use a clock at all. I have been reading up on some books the last few days so I am learning more and more. Very interesting stuff!