Inorder to make the update almost every frame (and thus 100% smooth as no frame has it in the same position), the text has to move every 0.01667 seconds (as the average FPS is 60). However in the new patch the FPS might be set higher or lower depending on the capabilities of the screen the user is using and sadly that number can not be fetched via triggers.
I would however recomend every 0.03 seconds or even 0.05 as the human eye is incapable of telling that it is not being rendered every frame. By increasing it to that number it uses less processor time and thus will make less lag when you use a complex spell or have a lot of background triggers running.
Remember that you are limated to at most 100 text tags (floating text) objects at once (after which the last created one will always be passed as the newly created one.