The event for warm up time is event number 2822. The message is "Warm up period complete." The eventlog.next() command returns the oldest event in the log, so you could return the event code:
eventNumber, message, severity, nodeID, timeSeconds, timeNanoSeconds = eventlog.next()
And check if the eventNumber is equal to 2822 before proceeding. Once you've read the event remotely, it cannot be read again remotely as it's cleared from the queue, but it does still appear on the front panel. If you restart the DMM, the warmup timer would be restarted and a new warm up event would be generated then.