I am successfully using the managed Lotus Nodes wrapper in conjuction with the Lotus Notes C API from C# to get scheduling information about all agents in all databases.
What I want now though, is to get information about those agents' run times. I'm thinking it must be in the API somewhere, but I've looked here http://www-12.lotus.com/ldd/doc/domino_notes/8.5.3/api853ref.nsf and can't find anything.
An alternative would be to see if there's anything in any log files that indicate start/stop times, but I don't really know where to look. There doesn't appear to be anything in the log.nsf.
Agents have an own log, that states some information about the runtime. You can get it via Designer via right- click and "View Log". For this information there has to be an api- call but I could not find it. One possibility would be to get the Note defining the agent and export it as DXL. In the resulting XML you will find the <runlog> tag that shows exactly when and how long this agent last ran.
Another possibility would be to set LOG_AGENTMANAGER=1
This would make every agent print its start end its end of execution in the server log.
You could react on these events using an eventhandler (configured in events4.nsf) and extract the runtime information from there.