How fast would the needs depreciate over time? Can Sims fast? That would be so cool, if Sims could fast and live an ascetic lifestyle!

I don't know about the rate of depreciation, but NRaas Relativity can increase or decrease that rate and one of NRaas Story Progression module allow you to make one or many needs static. That means you can make Hunger static and your sim and/or your townies never have to eat anything.

These are the rates for a young adult Sim with no traits:

Hunger = -11.25/h
Bladder = -13.33/h
Energy = -10/h
Social = -2.78/h
Hygiene = -4.76/h
Fun = -7/h

Data are taken from a game with AwesomeMod which gives numerical values and depreciation rates of motives.

Some traits, skills and lifetime rewards can change these rates. Childish Sims lose fun quicker (-10/h) while Sims with No Sense of Humour lose fun much slower (-3.5/h).
These rates also vary with a Sim's age. Social will decay faster for infants and Fun will decay faster for children, for example.