Reddit experienced some issues with Amazon a month ago that resulted in the site being down for almost a day. I'm pretty sure they're way below that percentage.

That's conflating SLAs again, Reddit's long-running problems have been with EBS reliability.

Even so, .5% downtime per year is about 44 hours, and Reddit's definitely had more downtime in the last few months than that.

Of course, that's assuming that 100% of Reddit's problems were due to EBS only and not a combination of EBS, EC2 and their own code.

Is this really standard practice for measuring the SLA? The contracts I've seen for a couple small businesses are generally per billable period.

Which always made sense to me. I pay you for 99.5% uptime this month. If you don't achieve it, then I get a discount, as simple as that. If your availability is below that, I don't pay full price for the billable period and then reconcile at the end of the year.

Any links or general advice on this topic anyone has I'd be pretty interested on finding out if there's a general consensus of it being done differently?

Don't forget to take in to account ALL of Reddit's downtime; there is quite a bit of it.

Which means their entire quota for this year is all gone.

and then some...