They say two month's salary is a guideline. I think that's for people who don't make as much as we do.
It also depends on your future spouse. Is she someone that needs the bling?
When we got engaged, I picked out my ring after he brought me a single diamond. It's got channel set diamonds on the side of the center stone and I went with something a little bigger than a half carat for the center stone. Any bigger and I'd break it off all the time. I'm a gardener and I'm remodelling my whole house by myself. I'm usually neck deep in drywall, dirt or grout around here. I can't see myself wearing a huge rock. It's a nice thought, but not really practical.
I think that our set (his, my engagement ring and the wedding band that comes with it) was about $3500. We went with gold instead of platinum, but if you wanted platinum that would bring it up to about $5k