One easy way to provide a paid vs. ads system would be to streamline under a single business model.
One example would be a user generated content site that allowed users to "be their own advertiser", and pay in proportion to their traffic/clickthrough/CTA value.
Unfortunately, while this may be easier for you as a company/accountant/developer, users have indicated pretty clearly that metered billing is too damn confusing, for the most part. One possible way to alleviate this is to offer the site as free with ads, but advertise at an inverse rate to the user's "ad value" – thus, no one ever has to "pay in", but you still provide a cleaner/less disturbed experience for your highest-value users.
Come to think of it, plugging in a site visitor's Klout score/other reputation metric, and then turning off ads for people above a certain level of influence might even be a generally good idea, as it removes all distractions from your site/content/design from the people most likely to spread the word virally to the mainstream follow-on visitors you will actually monetize.
Since the British are secretly holding onto Michelle Obama's Scottish Birth Certificate, there was general agreement to just go low-key on the matter, and hope no one notices…
A lot of this depends on the thickness of the blade, and the hair it's cutting. In my experience, regardless of the tactics I use, I notice a significant difference in blade sharpness immediately after the first use if I've grow out my facial hair a bit. This is pretty consistent with the guys I know with thicker hair who use the same/similar cartridges. For finer hair, I'm sure the techniques Dev mentioned will work to increase blade lifespan, prevent rusting, etc.
It's very similar to having a set of professional chef's knives – it's MUCH easier to notice when they start to go dull from being freshly sharpened (0-25% dull), than to measure the loss in sharpness from then on (26-100% dull). In fact, someone unfamiliar with the knives may even think they still "cut just fine", without realizing how much arm strength they are putting into the cut, or how ragged the cut ends of their meats/veggies/etc are, compared with a fresh knife.
The real issue is that there are few effective means to resharpen newer disposables. I recall there being a product on the market that worked really well with dual-blade cartridges, but I haven't seen anything that works with 3-4 blades (which is what I use now).
I would tend to measure in terms of "shave weeks/blade". The main variables are the thickness/amount of hair to be cut, and the regrowth rate.
Personally, Mach 3 gives me 1 shave week/blade… Whether I shave daily, twice a week, or even just once, the amount and thickness of hair compensate to basically make any additional attempted use of the same cartridge a game of "how much would you like to bleed?"
Just to give you some baseline info, I usually feel a little hair coming back 8-10 hours after a shave, and downright fuzzy the next day. I usually go 2-3 days between, though, otherwise I have a hard time getting a GOOD shave that can grab/pull the hair the right way as it slices.