Users do not really "log in" to Subversion in the sense that they establish a persistent connection & conduct all activity through it. A connection is established, if credentials are required to perform the activity they are requested & exchanged, the transaction occurs, and the user disconnects.
Many repositories are set up such that read-only activities allow anonymous access, in which case you can't track what you're asking for here.
In the case of serving via Apache, this can mean that a single checkout may appear to be hundreds of "logins" because each item appears as a separate entry in the httpd log. However, with a semi-intelligent HTTPD log parser, you can probably glean the information from those logs.
Subversion itself doesn't log such activity because simply looking at the repository or checking out from it doesn't change the state.