Rituals have several places time matters.
First, a ritual takes much longer to perform than a regular spell. Almost all spells can be cast in a single round, the base time for a ritual is an hour. Some rituals might be shorter and take only 10 minutes (1 turn), others might take a watch (4 hours), a day, or longer.
Second, a ritual might only be possible at certain times. A ritual that can only be performed at sundown or sunup obviously can be performed only once per day. The Ritual of the Tarnished Shield can only be done during the three days of the full moon.
Third, a ritual might have a lasting effect that lasts much longer than a normal spell can, potentially months or years. This could be simply a persistent effect, a transient magic item, or something else. The Ritual of Darkened Silence gives the recipient Clairvoyance for several weeks.
Fourth, a ritual might temporarily imbue an item with an effect, usable a limited number of times, and that expires after a certain period. The Ritual of the Waning Moon imbues a crystal ring with a single-use Invisibility spell, which dissipates within a year if not used.
Ritual times are largely symbolic, and scale in fairly large jumps.
- Round (1 round)
- Minute (1 minute, 10 rounds)
- Turn (10 minutes)
- Hour (6 turns)
- Watch (4 hours)
- Day (6 watches, 24 hours)
- Week (7 days)
- Month (about 4 weeks, 30/31 days, etc.)
- Season (3 months)
- Year (4 seasons, 12 months)
- Lustrum (5 years)
- Generation (20 years)
- Century (5 generations, 100 years)
Some of these have some leeway. A ‘month’, in ritual terms, is more likely to be tied to a phase of the moon. A ‘watch’ might not be the best fit for an ocean-oriented ritual, which might use a low or high tide (approximately two of each per day in most coastal regions on Earth — actually two per 24 hours and 50 minutes). A ‘generation’ might be 25 years and the ‘century’ replaced by a unit of 125 years — 5 generations has a nice symmetry, and 125 years is one-eight of a millenium.
Each step on the chart above adjusts things by one step. A ritual spell taking one hour instead of one round is three steps down the table, giving three ritual points to be spent on effect. A ritual that can be done only once per month (on the night of the full moon, say) might be considered two steps down the table from ‘day’ (assuming spell casters in the field usually aren’t willing or able to cast a particular spell each day) or three from ‘watch’ (because I’m pretty sure no spell caster can cast a particular spell six times per day without burning higher-level spell slots).