Tradition says it's two month's salary. However, there's no law that compels this.
If you're getting engaged, you and your SO should discuss what's to be paid for a ring. To be frank, it should be very telling of her if she insists on the two-months thing if you can't easily afford it. Going in debt for a ring seems to me stupid. If she insists, this will tell you what kind of woman she is and the level of importance she places on spending money and debt. And you should consider her response and think about the kind of marriage this might mean for the two of you...
As a personal note, my wife and I picked out the ring together and went with something that was affordable. I did not go into debt for the ring. But since I am a man, after all, I have no recollection of how much I did end up paying for it. LOL.
For wedding bands, she used the wedding band of her deceased mother and I use, and still wear, the band that my late father wore while he was married to my mom.
JJ
