Steve you have to make your own choice on this, there are two schools of thought about this subject, one is what is becoming called the ''optimists'', mostly economists, financial consultants, etc....... they believe we have no problems and as the price rises we will find more oil and then you have the ''pessimists'', mostly geologists, oil field engineers etc who say we are at peak oil about now and oil will become more and more scarce (and expensive) from now on.
You will just have to do some reading and thinking, my take on the subject is that the pessimist statements seem to make sense while the optimists seem to be based on wishfull thinking. Google ''peak oil'' or ''resource depletion'' and read on.
As to when it will get scarce, well that would depend on where you live and how rich your country is, if you are living in a country with very few reserves then it is already becoming scarce, try looking up the current state of Zimbabwe which cannot afford to import oil now at the current prices, later it will some other country and then another and then eventually it will be scarce in France and the UK and finally the only two countries left with the size, power and military to fight over the remaining oil in the planet will be the US and China. bearing in mind that they both have nukes etc... it might not be a nice battle, expect it around twenty years from now if the pessimists are correct.