I did the reverse for about 4 years, LGW neck of the woods to Oxford, on a motorbike most days and even then it was rarely 90 minutes or less, that journey and specifically the M25 I am afraid will break you if you have to do it on a regular basis, it seems it no longer matters what time of day or night it is the M25 will be queueing somewhere between J16 and J7, and it is very rarely a stress free trip. In a car I would allow approx 2 hours plus depending on where in Oxford you are based.