Price for the job not for the time it takes. Some jobs will be quicker some will be slower and all are subject to the unexpected. Dot sell yourself short you are providing he client with a service and it's easy to be too cheap.
Small jobs tend to be disproportionately expensive as your "standing" expenses are the same ( traveling, van, tools etc ) so form a larger percentage of the overall cost. Plus the fact that the "2 hour job" has actually cost you at least half a day by the time you've loaded your van, travelled to and from, collected materials, etc,etc.
I do agree with most comments on here . But if the job takes significantly less time than you allowed for and you have something else to go to ie half a day instead of a whole day and you have another job you can go to and still earn the rest of your days money . I would not have a problem charging them half a day particularly when trying to develop your business . They are then going to tell their friends how reasonable you were and you actually charged less than you quoted.
I'm still pretty new to this but if you're anything like me youll probably find that you will under quote your your first few jobs and they'll take longer than expected- It must have taken me a good 5 or 6 before I got my quotes about right (maybe I'm a slow learner).
I always keep to my quotes - sometimes they take less or more time but if the jobs good and the price doesn't go up then I can't see you getting any complaints - Good luck