It depends on how much you use your computer. I use mine off and on all day long.
I set the monitor to go on standby at around 20 minutes. It comes back on almost instantly. Leaving it on all the time wastes electricity and I figure it also wears out the monitor.
Having the hard drives spin down after 10 minutes is too fast, unless you only use your computer once or twice a day. Hard drives only have so many spinup and spindown cycles before they wear out.
My computers either go on standby or hibernate after two or three hours. Leaving the computers on all the time just wastes electricity. Mine resume from standby in a few seconds. Hibernate doesn't take much longer.
I rarely turn off my machines manually, unless I'm going to work on the hardware. I generally have no idea when I will be using them again. Having them go on standby automatically after a couple hours of idle time seems the perfect solution.
When I first built my machines the standby didn't seem to save much energy. The CPU still threw off heat, all the fans kept turning, etc. When I replaced the power supplies with Seasonic S-12, standby started working like it ought to on my machines with Asus mobos. No CPU heat, no fans turning, no power to the hard drives. Hard to tell they aren't powered off, but they come up where they left off in a couple of seconds.
Unfortunately automatically going on standby or hibernate isn't very reliable. Sometimes they don't do it. I haven't been able to figure out why, although I have a few suspicions.