Which file contains the code pertaining to how quickly diplomatic relations decrease?

It's giving me the shits having to send diplomats to every factions' city just to keep up a decent alliance with them. I've modded the AI to keep stronger alliances as long as the rating is </= Very Good but that's near impossible to keep without spending half my income on tributes to other factions just to keep an alliance.

Plus at 6 months per turn it doesn't seem realistic to me to have to continually pester the AI with diplomats.