What are the spawn rates for any given spawn point?

Solution 1:

Pokemon spawn at my house once an hour at precisely the same minute of every hour in exactly the same location.. For example, rattatta at 1:56 pm, krabby at 2:56 pm, pidgey at 3:56 pm and so on..

Solution 2:

Similar to what others are saying, the exact time seems to be variable. However, if you want to take a closer look at this, I recommend using Pokevision. This website can help you find Pokemon near your location and gives you the despawn time of each Pokemon.

From my experience, I notice some spots being popular for specific types of Pokemon. An example, is at the end of my street there is a house that always has water Pokemon spawning in their backyard. I've seen Krabby, Horsea, Shellder, Poliwag and even a Lapras once!