Cherry picking one of the few morning peak-hour city-bound express Frankston trains. After 8.40am, city-bound Frankston trains stop all stations and that same journey takes 43-49 mins.
If we're talking Cheltenham to Monash Uni then it's at least 40 mins during peak and 50+ mins off-peak. Not to mention needing to make 2 changes of transport at Caulfield and then Huntingdale.