If your site is down for a few days
This means that a site that went offline only for a few days due to some kind of a technical difficulty wouldn't experience a negative long-term impact in terms of search rankings.
If your site is down for couple months
In my case, I took my website down intentionally for 6 months after having it up for a few years, while it was passively garnering around 10K to 14K in monthly visits. I made this decision partially due to a work commitment and other personal reasons at the time, but on hindsight, this was not something I should have done.
Here is a screenshot I took in Google Analytics showing the flatlining of traffic I experienced during this ±6-month period, after which I resurrected the site.
And here's a summary on the traffic levels I received before and after my site went offline:
- 2–3 months before the site went offline: Around 350–450 daily visits
- 2–3 months after the site was resurrected: Around 150–200 daily visits
- Around 6 months after the site was resurrected: Around 300+ daily visits
- Around 8 months after the site was resurrected: Around 400+ daily visits
It was a fortunate thing that the traffic wasn't all lost, but based on my observation in Google Analytics, it did take around 6 months for ±75% of the traffic to recover.
But, also keep in mind that after the site was resurrected, I was adding quite a number of new articles to the website. It's highly likely that a number of articles I used to rank for no longer did, and some of my newer articles start ranking and that resulted in the incline in traffic later on.
Over and Out
Based on the traffic level, I delayed my site growth for around 14 months by taking it down for around 6 months.
I did not lose only traffic, but also rankings on pages that had gotten traction.
Some of my top-performing articles still managed to rank after the site was resurrected, but what took most of the brunt was articles that were just starting to get traction.
Moral of the story: If it is possible at all, don't take down your site intentionally.