There is no such thing as a guaranteed result. It’s either a positive or negative finding according to the case requirement.
Positive results indicates the presence of strong and relevant evidence on the person or company pertaining to the case objective during surveillance.
Negative results may imply that while under the surveillance time frame, there is insufficient evidence (or none at all) that is relevant to the case objective.
Usually a good private investigation agency will know where and how to find the required evidence and quote you an estimated timeframe that might be required for your case, depending on the nature of it.