I'm a landlord in PA. This is pretty straightforward and really isn't "a tough question to answer". The quick answer is "so long as adequate written notice was provided per the law in PA, there is no limit to how much is allowed". We have to assume that you are not living in a rent-controlled apt? If you are then that's a different story, but I'm guessing that this isn't the case.
So - IF you are not living in a rent controlled area then, there is no limit, provided that the landlord has provided adequate notice. This notice should be given at least 30 days in advance of the end of the lease. If you do not have a lease, or if the lease has expired and you haven't signed a new one, then you don't have a lease and you are simply on a month-to-month agreement.
If you believe that the landlord is raising your rent for an illegal reason (discrimination, etc), you should write a letter saying that and keep a copy. You can then refuse to pay the rent increase. If you do this, the landlord will probably try and evict you for nonpayment of rent. It will be on you to prove the landlordâ€™s bad intent in court.
On one hand, your landlord may simply be trying to maximize his profits, and the market may have changed quite a bit since you've been there. On the other hand, maybe you have been late, etc on your rent a fair # of times and he wants to get rid of you? Anyway, good luck!