I interviewed for a position a few weeks ago and at the time was told that the job title was for an HR assistant and that the position was non-exempt (hourly). However, after starting the job last week, they told me that my job title is HR administrator and that they will probably make my position exempt (salaried) because I am qualified. I'm not sure why they think I'm qualified to be exempt as my HR experience is limited to 7 months. I did previously work in marketing as a manager... perhaps that's the reason that they believe I'm qualified to have greater responsibilities as an administrator...?

It seems to me that the expectations from an assistant to an administrator are quite different. As a non-exempt employee, my employer has stressed that I not work overtime. However, the 3 other exempt employees (CFO, HR Manager, Financial Analyst) in the office all work more than 10 hours a day, 5 days a week.

Is my employer required by law to provide me with a pay increase, especially since I will very likely be expected to work quite a bit more than the 8 hours a day, 40 hours a week I'm currently asked to not exceed? If not required by law, is it reasonable for me to ask for a pay increase in expectation of working more hours? If yes, any ideas on what percentage is reasonable?

Thanks, in advance, for any answers anyone might have!

Is my employer required by law to provide me with a pay increase

Assuming that you will be earning more that $455 per week, no.

If not required by law, is it reasonable for me to ask for a pay increase in expectation of working more hours?

It is not unreasonable; however, they are within their rights to say no, and if they do, your choice is to accept their decision or find other employment. Nothing in the law gives you any means of forcing them to provide a pay increase.

any ideas on what percentage is reasonable?

None whatsoever. I'd have to know FAR more about your job duties, what you are currently earning, and what an equivalent job both in your company and in your market region earns before I could even make an educated guess.