Rituals are about control. In a slow and tedious process, magic is woven into exactly the effect that the wizard desires. Anything is possible using rituals, it is only a question of time and effort. Even effects that are beyond the powers of spells can be accomplished through rituals.
Rituals take time and often the only way to reduce the time to acceptable periods is through the use of components.

To estimate the time required for a ritual, calculate the equivalent power level just like you would for a spell. However, rituals can go beyond spells in power, so there is an additional level with a multiplier of 10:

Effect

godlike effect, beyond anything that mundane means could accomplish

Target

anything within view, several miles in diameter or - for spells with a distance rather than an area of effect - up to the horizon

Duration

years, a lifetime or permanent

As with spells, multiply. Yes, this can bring rituals up to an equivalent power level of 1000. Obviously, no wizard is likely to have enough skill levels to accomplish this - if skill levels would limit rituals. Fortunately, they do not. As long as you have at least one level in the Intent and Base used, you can try the ritual. But skill levels matter...

Time is the important factor for rituals. To find the base time for a ritual, simply divide the square of its power level by the lower of your Intent and Base skills. This gives you the number of hours that the ritual will take.
So if you have Fire 4 and Harm 5, you can't cast a Fireball spell, because your skills only add up to 9, but Fireball is a Power Level 12 spell. But you could do a ritual. It would take 12*12 / 4 = 36 hours.

Fortunately, components can reduce the time required for rituals dramatically. When components are used, the effective Intent and/or Base used for time calculations can be raised, resulting in reduced ritual times.
The components must fit to the skill they serve to raise. Here are some sample components:

Followers

Having a choir or helpers can add to Control, Mind, Change and Perceive.

Offerings

Destroying valuables, gifts, etc. can add to all elemental bases if the offerings fit (e.g. plants for Earth, wines for Water, etc.)

Sacrifices

Killing animals can add to Harm, Death, etc. Human sacrifices count as two animals each.

Visibility

Preparing a place with runic markings, having drums or music at the ritual and other efforts that draw attention can make most rituals easier.

Use the square root of the number of components used, rounding fractions mathematically.
So imagine you you want to raise both Fire and Harm to 6 for the above ritual, so the time required drops to 24 hours. You could sacrifice an animal to raise Harm by one point, and burn at least 4 candles to raise Fire by 2 points.

Obviously, reducing ritual times to enjoyable values requires either a lot of resources or a lot of skill, a combination of both, or something that many wizards have difficulties with: Teamwork.
If several wizards work together on a ritual, they will add up their skill values. Still, the lowest total is what matters.

One advantage of rituals is that they don't use up concentration. Instead, they will stop the regeneration of concentration for the time you are busy with the ritual.