It's been 20 years since I've been through the Mexico City airport, so I'd be grateful for a little help here. The flight schedules are such that my most convenient itinerary from SFO to Merida and back from Cancun would be to take United roundtrip to/from Mexico City, and nest in an open jaw RT on Mexicana to Merida and from Cancun.

So, what about this? How much time should I allow at Mexico City, given the need to go through immigration/customs? Any other thoughts/comments on this plan?

Thanks in advance for your time considering this.