There is so many variable answers that I could answer this vague question with. tiling with 6x6 white is going to be faster than tiling with 600x600 porcelain But that is pending what your tiling. for example tiling a 3 metre kitchen splashback with 8 sockets and metal trim is going to take far longer than tiling a square room minimum cuts with 600's.
As far as I can see from what I have read. If I quote a customer a fixed price, that is it set in stone. usually with my estimates I am not far off of my timescales, i.e if I say two days, it is two days, so on and so forth. I very rarely get arguments over price(s) after starting a job but if I do I remind them that we agreed a set quote. If said two day job had taken four days I would still charge them the same price, as that is what we had agreed. The only exception to that rule is if they add on other work, more tiling, plastering, plumbing etc.
ultimately sometimes we get it right, sometimes we get it wrong but the pendulum swings both ways. I have seen myself on "simple" jobs that have turned into living nightmares and in the same instances I have seen myself start "nightmares" that have ended up into fairy tale besting jobs ever.
Tiling is like a box of chocolates..... haha