Historically there’s been a major outage affecting most customers in a region about once a month for each of the last 18 months except one, but then a couple of months had more than one major outage. The major outages do generally appear on the status page.
In addition, every nine or 10 days there’s been a minor issue which will affect only some customers depending on their specific configuration or the devices they are using or sometimes just a random percentage of people get hit. You can see these on the “first bug reports” page in the community – created wiki. Sometimes these have a workaround, sometimes not – – the bug report will usually link back to a thread in the forums where people are discussing options. Some of these “minor” problems only last for a few hours, some have lasted for months. 
http://thingsthataresmart.wiki/index.php?title=Bug:_First_Reports
All of these are separate from the planned outages when smartthings pushes out a hub update. The planned outages can neither be delayed nor denied, and there’s typically at least one a month. We usually get a few days advance notice on those, but not always.
The following thread may be of interest (this is a clickable link)
So different people have different experiences, but an MFOP (maintenance free operating period) Of about 15 days is probably typical.
The company continues to say that they hope to make reliability and stability improvements, but they’ve been saying that for two years and statistically the number of outages doesn’t seem to have changed much. 