While page loading speeds [both frontend and backend] vary from site to site, 30-40 seconds is extremely high, and beyond what I would personally consider normal.
There are lots of reasons that your admin could be sluggish. If your error and exception logs don't show any issues that need to be resolved, it could have to do with an extension that you've installed. One way to diagnose that is to disable different extensions to see if that resolves the issue.
There could be a hosting configuration issue. You can reach out to your web host for guidance on that.
Beyond that, there are tools like New Relic that can help you look for issues that could be tying up resources or erroring out.
Back to your central question - it's not easy to specify an exact loading time that a unique site should have, but under 10 seconds would be expected for most common admin pages.
Of course, if you're pulling a large report, or doing something that's particularly resource-intensive, speeds will vary accordingly.
