Your landlord has a duty to maintain the apartment in reasonable repair such that it is fit for habitation. An issue such as a broken window should be resolved reasonably promptly.
Rather than simply withholding your rent, I would advise you to notify your landlord in writing that you intend to place your rent in an escrow account commencing May 1 if the window is not repaired by that date. You should include in your letter all of the dates that you have previously contacted the landlord about the window.
On May 1, if the window is not fixed, you should place 100% of your May rent in an escrow account, and provide proof of the deposit to your landlord. The funds should remain there until the window is fixed. Placing the rent monies in escrow will provide you with protection if the landlord decides to take you to court for nonpayment of rent.