There’s nothing wrong with AUSPOS. I use it multiple times a week and have for many years along with other GPS/GNSS services and various post processing software like RTKLIB.
For the original question a service like AUSPOS or CSRS-PPP is best if you don’t have long baseline GNSS processing experience. How long should you observe? AUSPOS suggests a minimum of 6 hours. This is for a receiver which has dual frequency measurements on all GPS sats. A L2C only receiver won’t - so up the observation time a bit. The sats without L2C can “bunch up” making certain time periods of an hour or two not so great for dual frequency GPS.
Use both services and compare the results. A half a day of data is a reasonable time period for cm-level results. Obviously more time will increase your confidence level.