You don't! You can cast 50 spells a day or just one! As long as you have the energy and are following the instructions and doing what is required. If any spell says it will work within a significant amount of time, then that is NOT true! The effectiveness of the spell depends on the caster and on how well you have followed the instructions and did what was asked or had the required ingredients. As to when a spell will become effective.. that no one knows, a spell will work when it wants. But there is not time limit to how many spell you can cast or have to wait but do refrain from casting the same spell over and over.
Well this is very simple!! To answer your question, you can cast as many spells a day as you want although it will take alot of energy. Also it takes a long time for a spell to work certain ones sometimes take shorter. If you cast a spell even though it may say when it will happen it might not work. Spells take their own time, and you have to know what your doing because it depends all on the caster! hopefuly this helped
You can do any number of spells per day as you wish.
However, you should know that if you are performing evocations, it is considered rude to evoke a different spirit to request the same task as you have recently requested from another spirit recently.
A rule of thumb is to wait atleast a month before evoking a different spirit for a task already asked of another spirit if no results are seen.
Note that evocation of and asking spirits to aid you - there are results 100% of the time. The only time results are NOT seen is when it turns out the chosen spirit is not specialized in the area pertaining to the requested task.
Think of spirits as you would a tool - use the right spirit for the right job.