I was looking into that when writing SystemDataScope/collectd modules. Unfortunately, I didn't find any exported stats on SOT. There is current screen state exported via stateFS, but its showing only a current state of the screen. For long-term stats, its pretty much useless if you want to collect stats via rare sampling. Alternative is to write a service that would hook itself to dbus and follow the messages sent around while the screen state changes. That's not too simple with the common tools exposed to collectd modules, so I didn't look much into it.

So, if you know how can I determine from system files (/proc, ...) for how long the screen was switched on, I'll be happy to expose the stats on it via collectd and SystemDataScope. Other major missing stats is GPS state, so suggestions on that are welcome as well

Alternative is to write the services collecting SOT and GPS states and merge them to Mer's stateFS system, for example.