I can not say exactly. It should be about 41 hours first class to Mexico City and another 24 hours to Guatemala City. Doing this all in one trip with only bus rest stops seems borderline crazy. The bus rest stops are generally less than half an hour, enough time to stretch your legs and eat some bus station food. If you chose lower class buses it could be a week and you could experience some of the discomforts mentioned by a previous poster.
Buying your ticket from Greyhound would be200 from San Diego and180 from LA. (Yes, it is cheaper to go from LA.) But I would expect a much better price in Tijuana. Still if bus travel costs 3 to 5 dollars per hour (I guess that this is the range for first class buses in Mexico these days.) then you are still talking120 to200 dollars.
You can fly from Tijuana to Toluca, Mexico with the (Free shuttle to DF) for 1400 pesos including taxes on Volaris Airlines (ie. about 135 dollars).
Good luck, bob
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