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2.5 hours is likely.
There's more information on transiting MIA here:
http://www.flyertalk.com/forum/ameri...onnection.html.
At MAD, ask if your bag can be checked "ITI" - MIA is one of the very few US airports where you can transit without collecting your baggage, taking it through customs and rechecking it. "Como mi destino es en México, quisiera etiqueta roja "ITI" para poder pasar por Miami sin tener que recoger mi maleta." / "As Mexico is my final destination, I'd like red "ITI" tags so I don't have to recover my baggage in Miami."
You'll still have to go through immigration and as it says in the Wikipost at the top of the page in that thread,
Official MCT / Minimum Connection Times for MIA (assuming same terminal):
0:40 Domestic to Domestic
0:45 Domestic to International
1:35 International to Domestic
1:35 International to International
NOTE: Though 1:35 is the given MCT International to Domestic (and 1:35 international to international), one should not consider that useful for everyone connecting international to domestic unless one has Global Entry or luck.
International arrivals at MIA requires most passengers to fill out a U.S. Customs and Border Protection Declaration Form 6059B - one per family - process U.S. Immigration, retrieve baggage at arrivals carousels, process through U.S. customs, re-check baggage, proceed to either the TSA connections security processing queue or landside and process TSA security (with onward boarding pass) to return airside. (Read on for exceptions.)
This does not apply if you are connecting international to international and have red "ITI" tags on your checked baggage. You will have to produce the Form 6059B and still pass through USCBP / customs, albeit without baggage.