How much alimony could I expect to pay

After dividing marital property my ex and I will walk away from marriage with 250 thousand each, no debt plus she will recieve 40% of my military retirement, which would amount to 1200.00 per month. My gross income is 95 thousand after paying her the 1200 per month. I have custody of our son. I live in Pa and we were married 18 years. She worked in beginning of marraige but then quit and never returned. She has a bacholers and is in school to renew some classes. She is 52. No children at home. Thank you for your time.