How long do you have to file with workman's comp?

I know I could google, but I did and can’t understand the legal jargon.
I was injured at work on January 11th. I waited to see if it got worse, because I thought I was just in pain from the fall. It’s been two months, and I am having trouble with my knee. When I fell, the employer told me if I need to see a doctor, to just let them know. I’m worried that so much time has past, that they will say it’s too late.
This is the site I found on it, in my state http://www.workerscompensation.com/regulations/stateitem.php?ID=19040&state=pennsylvania&Parent=1792&title=
Is there a set amount of time that I can no longer file a claim? Should I inform my employer before I see a doctor? I do not have my own insurance, so it’s not like I could use my own.