There is no standard amount. The marketing arm of DeBeers and the American Retail Jewelers organizations suggest three months gross income for working individuals.
You should only spend what you can comfortably afford. You should also take into account the norms in your community. If your kallah’s classmates, friends and relatives sport 1 carat diamonds, don’t buy her a 4 carat stone, even if you can afford it. She will feel awkward wearing it and attracting attention.