As someone who has also not long started out in tiling, I admit it is a hard one to gauge. I am never going to be as quick as someone who has been tiling for years, so I've for all the jobs I've kept my prices lower but been honest with the customer that the reason for doing so is due to the inconvenience of me being in their house for longer.
The third job I done was a 5m2 kitchen splash back, which was a referral from another tiler. He said it should only take about 1 day to 1 1/2 days to do, but took me 3 - mainly because the walls weren't the best and I was wanting to make sure the finish was perfect and I used tubbed adhesive rather than rapid setting adhesive.
All I will say is that every job I have done, I have probably made a loss in comparison with more experienced tillers, due to the length of time I've spent, but for me the experience of doing it and doing it right is more important to build a solid portfolio.
To answer the original question, I price for preparation at a per hour rate, larger floors and walls at a m2 rate and kitchen splashbacks at a day rate/half day rate, depending on how long I think it will take (which is sometimes much longer than I estimated - still, I'll get better with experience).