We've done a drop-off with
Alamo a number of times. Unless there's a huge rush of cars being returned, it really should take 5 minutes to enter the garage and hand over the keys. Then it's perhaps a 10-minute walk to the counter to check bags (if you're checking any). That being said, I have zero experience with Terminal C, just A&B. If you're flying from Terminal C, I don't know how much extra time that might add.
If you're A or B, you can actually drop off the car at either drop-off point, A side or B side. If you drop-off in A but are flying from B, it might add a couple minutes to walk across to B (as an example), but it's very easy to do that. But I've found it easiest to return to the first location I come upon.....