What’s the cheapest time to ride an Uber on Sunday’s?

It's difficult to predict. It depends what events are going on, the weather (where I live, we have a few short months of comfortable weather; nice days cause people to travel around more), unemployment benefits for the drivers, etc. Some services allow for making appointments for rides, and I found that the charge at the time of booking the  appointment was the same as the charge for the ride.  

i would think its more the distance that makes it expensive and not the time

It depends how popular the destination is and howd the weather

it might depend more on how far youre going than what time youre going