How much time should I allot to Teotihuacan (taking a bus from Mexico City)?

You might as well see the entire exhibit: Pyramids of sun and moon, house of jaguars, temple to Quetzalcoatl/Tlaloc, la ciudadela, the onsite museum, the calzada de los muertos, and all the lesser ruins. You might want to rest a couple times while at Teotihuacan, especially if you are not accustomed to the altitude here. I would say give yourself about 3 to 4 fours at Teotihuacan, plus the bus ride out and back.

Depends entirely on the day you're traveling.

I say if you go from Monday to Thursday you'll do 2 hours.

On Friday probably 2.5 hours, and on the weekend 1.5 hours.

You have to separate 5 hours; 6 hours at the most.

Half hour to get to bus station
1 hour to get to the pyramids
2 hours in the pyramids (bring sunblock!!! and a cap!!)
1 hour to get back the bus station
Half hour to get back where you wanna be

Total: 5 hours.

Enjoy!

You're going to Teotihuacan? I'm so jealous right now. I can't wait, I'm going this winter to Mexico.

If you go to the Terminal del Norte, it will take you about an hour from there. I'd plan on spending at least 3-4 hours there and taking the bus back, another hour. Before you leave the bus station in the morning, find out when the last bus leaves Teotihuac谩n to return to D.F. (Mexico City). Make sure you bring sunscreen and a bottle of water. It is the rainy season now, so you might want to bring a small Totes umbrella as well. You'll be doing a lot of walking at a high altitude. Unless you're from Denver (and already used to the high altitude and rare air), plan on taking it slow and easy. There are a couple of restaurants outside of the pyramids that are pretty good.
Make sure you take the bus to the pyramids and not to the city of San Juan de Teotihuac谩n!