Not an expert at all but we have been thru Miami customs close to 100 times. Sometimes you arrive and there is no line at immigration other times you have 25 to 50 people in front of you.
A usual 10 to 20 minute wait on baggage and usually the line at customs moves pretty fast.
The big holdup is getting back in line to go thru security again to catch your connecting flight. The lines here can be pretty long and I think that the max we have ever stood in this line is another 20 minutes.
I would be nervous if I planned to catch my connection with only an hour in between. I also would plan on an hour and a half to two hours.