Q:

How can I tell how long Solr replications and commits take

I know that my Solr commitWithin should be set to a greater length than a commit usually takes.  
Likewise, in my replicated setup, I guess my pollInterval should be set to something longer than a replication takes.
I can't work out how to see how long commits and replications take though - how do I do it?

A:

When you send any request to Solr you get back the QTime within the response. That's the time spent on the server to process your request. If you send a commit you'll get the time spent on the commit itself.
If you're using auto-commit you can have a look at the Solr log file to see how long a commit takes. I'm not aware of any other way.
You can also have a look at solrmeter, it allows you to run some performance tests against a running Solr and it shows a nice graph including commit time.
Regarding replication you get some details passing the parameter command=details to the replication handler, but I'm not sure some replication time is there. If not you need to look at your log file again.

