i have used superthrive for about 5 years now.
it would be pretty hard to burn your plant with it. it is a hormone, so the plant reacts differently than it would with nutes.
if a plant is subjected to more hormones than it needs/wants, i will simply stop taking it up. but too much coupled with nutes can aid with nute lock, which of course can screw ur stuff up.
a drop per gallon of ST is usually the most they will take up, so putting more would just be a small waste and will just go stagnent in the soil and of course become inactive.
it is mainly a b1 supp. though, with little auxin. so using it during flowering wont help you out, and will only aid in foliage growth- as well as make it hard for it to take up any other hormone or supplements you may want to give it.
outdoor, sufficiently i use it no more than twice in a full life cycle. one feeding with it at the beggining of veg, and one in the middle of veg.
indoor, just once after a seedling or clone has got some girth on it. as is with most hormones, u never need much. and in most cases, dont want much.- can confuse and stress the little buggers.
i still have my original 2 bottles. it should last you a long time
good luck, superthrive is some good stuff