A Need is a specific thing or an action. There is no ambiguity, ever, about whether the Demon received its Need and when the last time was. The demon may like its Need, hate it, or regard it as a physiological necessity. What matters is that it's literally addicted to it.

Binding by definition makes the Sorcerer responsible for providing the demon with its Need. Both participants understand this in full, for any and every instance of Binding, even if the sorcerer is Naive. Failure to receive its Need makes the demon lose Power, just as a Parasite or Possessor loses Power when outside a host. Under-supplying or frequently-supplying the Need does affect the demon's tendency to rebel or not to rebel.

Desire is ideology, personality, taste, and preference. Need is addiction, payment, and power.

Ron Edwards

Some demons are capable of acting on their own to get their Need fed, but doing it for yourself (as a demon) just "isn't the same."

As a general rule, demons will always prefer to get their master to proactively feed their Need in some direct or indirect way... it's just better for them that way. [1]

When is a Demon in Need?

There is no regular schedule of demon Need-feeding built into the rules except for "once a week or so." The "or so" covers customized demon concepts originated by group that might lead to a different schedule.

On the other hand...

When is a Demon in Need?

Always.

What matters is when the demon is expecting its Need to be fulfilled. As a rule of thumb, figure a day for it to mention the Need, or at least perk up its ears to make sure it's coming soon, and a week before it gets irked enough for its behavior to be off-base.

A demon might usually appreciate immediate gratification upon doing something dangerous and especially upon performing something that doesn't accord well with its Desire.

Never permit "well of course" logic. ("Oh, well I always feed him a mouse before going to bed. That goes without saying.")

With Demons, _{_nothing_} "goes without saying". [2]

See also: Feeding Desire

Chapter Five of the Sorcerer text regarding the design of demons summoned during play:

The player describe the demon's Need, while the GM decides on the demon's Desire.