I presume it only applies once, as in every prior edition, and that there will be stronger poisons in phb or dmg.
I wouldn't presume anything that isn't written in the rules. They're trying to make this edition extremely user friendly for new players. When the text isn't clear that a rule works how we are used to it working from prior editions, there's a really good chance it doesn't work that way.
In this case, I expect it works for each successful hit during the entire minute.
I'm not surprised by the math. Even with multiple uses over a minute, it seems weak for $10,000, er, I mean 100gp.
The only time I can think of where it would be worth using is in assassination types of scenarios. A warrior isn't going to poison his blade before a battle. But a rogue lurking in the shadows might very well do so.
Overall, it looks like poison
could potentially end up being useful in this edition (depending on what other types they make), since without an expected wealth by level or magic-mart a character might be able to amass a lot of wealth, make his own poison, and end up with a reasonable supply that he doesn't have to give up permanent magical items to pay for.
Again though, it's generally only going to work well in specific planned situations--not as a standard buff.