Elemental infusion increases base damage of most weapons by 40%, split evenly between physical and elemental damage. There are a few exceptions (like Blacksteel Katana, Blue Knight's Halberd, Icre Rapier)

Without taking into account scaling (because it varies so much), an infused weapon breaks even with an uninfused weapon when an enemy has 40% elemental resistance. Most enemies outside the DLCs have a bit less than that. Exact defenses vary, but in general infused weapons come out slightly ahead of uninfused weapons... unless you get a high scaling bonus from a weapon. But infused weapons are nice because you really just need the base stats to get most of the benefit from them, letting you focus on leveling other stats.

If you also use weapon buffs, enemies need around 70% elemental defense for infused+buff to break even with uninfused and 60% for infused+buff to break even with uninfused+buff. Again, this doesn't take scaling into account.

Once you get to the DLCs, enemies have high enough elemental defense and you're at a high enough level that physical weapons with high STR/DEX scaling are better than infused weapons.