How does % purity on household products work?
For example, toilet bowl cleaner is 9.5% HCl.


First off, do you see "w/v" mentioned after those figures, i.e- $\ce{HCl}$ 9.5% (w/v)?


  Does this mean that for every 90.5 moles of H2O (or other stuff) there is 9.5 moles of HCl? Does it mean that for every 90.5 g of H2O there are 9.5 g of HCl?


Even if you don't, I'd still expect the concentration to be expressed in the form: $x$ grams of $\ce{HCl}$ for every $y$ units of water. As for whether water's mass or volume's considered, it would depend on the regulations for such products (which I'm unfamiliar with).

However, I wouldn't be surprised if it did turn out to be a w/v quantity (which is quite common in pharmaceutics).


  Kinda weird that we don't know if this is weight or molarity.


Like I said, I'm going to throw my weight behind "weight", since it's far more common than molarity outside of a lab (come to think of it, I've never seen a household product's ingredient concentration expressed in molars).

Not exactly important, but since you mentioned "molarity", here's a handy formula to convert something in % w/v to molars:

$$\text{Molarity} = \frac{{10}{(w/v)}{(d)}}{M}$$

Where $\ce{d}$ is the density of the solution, $\ce{M}$ is the molar mass of the solute and $\ce{w/v}$ is the mass concentration of the solute.