If an employer cuts pay, how long can they do this for and do they have to make it up?

Unfortunately, I think that you may be out of luck here.  Most employment relationships are what is known as "at will".  This means that basically the employer can hire or fire someone for any reason or no reason whatsoever, as well has increase/decrease salary/hours, promote/demote, and generally impose requirements as they see fit.  In turn, you can work for an employer, or not, your choice. 

As such your company is under no obligation to "give back" any salary cut.  Basically, the only exceptions to the above would be if there is a stated company policy covering this, or there is a union/employment agreement that governs, or this situation has arisen due to some type of discrimination.