Generally I go for it when I start wanting to fix my city happiness issues. I'm not a huge fan of hereditary rule (can't be bothered to make units for most cities that are nearing the happy cap), but forges are often a big happiness booster since they double the happy effects of gems, gold, and silver. The extra productivity is a nice bonus, of course.
I tend to end up putting it somewhere around when I'm going for Paper / Philosophy. That just seems to be where it ends up being convenient to pick up. There's no real plan behind this, of course.
Funny, I find that my games go the exact opposite way. I get Monarchy and then HR takes care of my happiness which makes it really hard to justify giving away techs for MC.
It is just so much easier to build an archer than a forge
