There is an abundant number of sadhanas in Hinduism. The nature of sadhanas can be classified into three categories: Sattvic, Rajasic, and Tamasic.
Doing Sattvic sadhanas are highly recommended for anyone. Non-Sattvic sadhanas can have dangerous implications and can spoil the entire life of a person and the aftermath may also continue after death.
Almost every sadhaka who started any kind of sadhana wants some kind of experience, primarily visual or auditory. If the sadhaka sees any being and communicates with the being, the sadhaka feels encouraged.
Unfortunately, in the spectrum of sadhanas, the sadhanas that fall into the left side, are known to give almost instant or very quick results. For example, in the sadhanas like Karna pisachi, the sadhaka can able to see and talk with the pisachi in a minimum time period of 3 days.
When Karni Pishachini is invoked, you would know. It takes a period of minimum 3 days for anyone to invoke Karni Pishachini and tame her to work for you. Source
The source may be weak, but I listened to the same information from multiple sources. Although sadhaka gets to experience it, the aftermath can be extremely dangerous.
Unfortunately, I never heard of any sadhana that is as quick as this sadhana but sattvic in nature.
Is there any sattvic sadhana that is done under proper conditions, so the sadhaka can experience the divine being in very less time?