Cronitor co-founder here. This is a great question! I would love to see others post some numbers here.
As Shane mentioned in the post 12.5% has been a consistent number for us as we have scaled up over the past couple years. That said, at this point I suspect we will see this percentage decrease a bit going forward. I'm basing this on intuiting that we're a bit over provisioned at the moment and won't have to scale our infrastructure linearly with user growth over the next year or two. Of course, that remains to be seen...
To add some more numbers to the conversation my full time job is CTO at a consumer facing tech company (Babylist.com) I just looked at our spending on IT/infrastructure for the last couple of months and it's around 1.5-2% of revenue.