Depends on how long it lasts and how popular niche your website is ranking in. For example if your website is temporarily offline for 5 minutes and you have a good uptime history, then you won’t have anything to worry about. If your website is down once a day for 5 minutes then eventually you’ll get penalised by all the popular search engines.
Something else I have witnessed in the past are sites getting penalised in highly competitive niches after being offline for no more than 20 minutes. If you are ranked #1 for the word “golf”, you pretty much need to have a perfect uptime history to maintain that rank. If your site were to go down for a few minutes, that could result in 10,000 bounces which is going to catch the attention of Google! You don’t want that.
In contrast if your site is ranking for “guelph SEO company”, that might only get 10 searches a day. If your site were to be offline for 20 mins, it might only result in a handful of bounces, if any at all. That’s not going to be sending alarm bells to Google, which is great as it gives you more time to recover.
The biggest challenge I have faced regarding downtime is when my website is temporarily taken offline due to my shared hosting account. I can handle having my website offline if I am planning site upgrades and things of that nature. What I can’t stand is when my website has gone offline because of what someone else is doing on their website that has nothing to do with me other than the fact we are on the same server and share resources through our shared hosting package. Web hosting companies will not notify you when things of this nature occur. It is up to you on how you handle, and keep track of these occurrences. My recommendation would be to get a dedicated server, that way you have control of the situation and don’t run the risk of your site going down due to what someone else has done.