The log message that always show up for oplog truncation, which can indicate when oplog truncation is taking a long time (high resource consumption, perhaps), with log-level 1 is:
"WiredTiger record store oplog truncation finished in: <elapsedMillis>ms"
A more informative message for debugging, which communicates an oplog size guess, might be the log-level 1 prior message:
"Finished truncating the oplog, it now contains approximately <sizeInfo_numRecords_load> records totaling to <sizeInfo_dataSize_load> bytes"
Or, better yet, we could combine them into one message at log-level 0.