It's interesting that the differences in the job role in the US and UK are not very great but when it comes to the notice period, there's a huge discrepancy in how it's handled.
For the Benefit of both countries, let me give you a low down on how it works in each.
In the UK when you are offered a position you are also offered an employment contract which defines the position and the salary and generally the all important notice period. Notice periods range from 1 week if you're not on a contract to some of the longer terms you've heard on this thread. And if the parting is amicable, it's generally not a terrible thing to give an employer as much notice as possible. The employer has the right to pay the employee in lieu of notice, for instance if he's an unhappy camper and wants you to leave today, if you've given proper notice then you're (generally) entitled to pay for that time period. It works out pretty well. AND the longer you've been in a position the more likely it is that the notice period will be increased in successive contract.
In the US, the employment contract is not as common (although I do know of very high end executive assistants who have them). Generally the company policy manual and a job description are relied on to set the rules of employment and in most cases notice periods will be spelled out in the P&P manual. In the US a two week notice is generally acceptable unless you've agreed otherwise with your employer.
Obviously there are individual company differences but that's the basics
