Help with girlfriend who wants to take a "break". How long is this to last and what do I do?

My girlfriend and I have been together for 1 1/2 years. This whole time we'd rarely fight and I felt that I always had the control in the relationship being that it seemed she'd never leave me. Recently, she's been working very long hours at her job and also has two certification exams to prepare for. All of a sudden, she tells me she needs a break which hit me like a brick out of no where. I have been doing everything in my power to gain her back and we are still seeing each other from time to time but it's just not back to normal. This is our first 'separation' and I'm not sure how much time would be normal to give her to herself as I don't want to lose her. Granted when I was with her I would have my doubts at times, but now that I lost her I know she's the one because I cannot fathom being with anyone else besides her and this is killing me. I keep trying to respect her wishes by giving her time to herself, but it's torturing me inside to leave her alone as I'm afraid too big of a gap would cause her to completely fall out of love.. please help.