Auch diese Frage wurde wohl im Lebenszyklus von D&D 3.5 geklärt.
tl;dr: Versorgungsringe haben keinen Einfluss auf die Vorbereitungszeit von Zaubern, tägliche Zauberslots etc.
D&D 3.5 FAQ Archiv, S. 59The description of the ring of sustenance says the user
gets all the benefits of 8 hours of sleep in 2 hours. How does
this affect a bard, sorcerer, or wizard who wants to regain
spells? Specifically, how does this interact with the casting
limit rule in the PH?In the case of a ring of sustenance, “all the benefits of 8
hours of sleep” means the character sleeps for 2 hours and
regains 1 hit point per level (see the rules for natural healing on
Chapter 8 of the PH). If the ring wearer is fatigued, 2 hours of
sleep removes the fatigue.
A wizard must have 8 hours of rest before regaining spells.
If the wizard doesn’t have to sleep for some reason, she still
requires 8 hours of rest to regain any spells (see Preparing
Wizard Spells on page 177 of the PH). A ring of sustenance
doesn’t change that.
A bard or sorcerer regains spells only once a day, and a
ring of sustenance doesn’t increase that.
A ring of sustenance also doesn’t exempt the wearer from
the casting limit rule. Whenever a spellcaster gets a new set of
spells, any spell slot she used in the last 8 hours is not
available. This rule has nothing to do with how much sleep the
spellcaster gets; it reflects how long a spell slot must remain
empty before the character can refill it. The ring doesn’t make 8
hours pass, so it doesn’t help the character refill the used spell
slot.