When is the right time to introduce high availability for web site?

Short answer: When down time or the risk of it costs you more than it would cost you to have high availability.

It is fundamentally an economic decision. As an example. $8k/month implies that an outage of 2 hours will cost you $22. If you can configure your system such that you can go from scratch to a fully functional site in 2 hours, then high availability would only gain you $22 of functionality above that.

Put another way, you can save money unless / until you have 54 hours of unpreventable down-time in a given month.

Your stakeholders/business folk (which could be you!) have to decide

• acceptable loss of revenue
• consequences loss of reputation, respectability etc
• acceptable data loss (Recovery point objective)
• acceptable downtime (Recovery Time Objective)

Loss of revenue is easy to quantify: the rest can't be answered here sorry...

I think most users can handle a bit of scheduled downtime. Consider that ebay has weekly updates on friday nights, and bids around then sometimes don't work. My (major australian) bank's online banking has scheduled outages for hours every week. Twitter goes offline all the time. Heroku / EC2 was down for days recently.

I'd keep it in that perspective, if you're really only talking 5 mins a month, you're doing quite a good job as a sysadmin.

You've already mentioned Google as a factor in terms of indexing, but it may also be worth considering the impact that latency/site responsiveness may have on SEO. It's a black box and all that, so difficult to quantify - though for what it's worth, Matt Cutts reckons it's a one-percenter. I'd be more concerned about reputation, as others have stated.