how soon has contact been observed

Keep in mind that contact can be a soft touch, slight breeze or even a soft whisper. It can also come to you in what you thought was a dream.
TV and Movies portray contact in a dramatic way but in reality it is as simple as a soft kiss

No, a certain amount of time does not need to elapse between a death and when they decide to make contact. In my experience, when a death is very sudden, the deceased will try to make contact right away, many times to the first person who will listen (especially if you are a Sensitive). After that, they will feel satisfied that they have said their "goodbyes" and will move on. On the other hand, when someone goes through a more prolonged death process, say for example, they have had terminal cancer and have gone through their "goodbyes" while still in the corporeal state, then they will move on to the other side and be processed right away instead of hanging on. In this instance, it might take weeks or months for them to return and make contact with a living relative. Again, this is from my personal experience and by no means the hard and fast rule to all visitations. All people and all spirits will do as they please

my clients have reported usually around the time of death or shortly after but also sometimes long after the fact.

So many people have paranormal experiences once a loved one has passed. My question is does anyone know if there is a certain length of time between the person passing & when experiences occur? Can they make their presence known immediately or is it after a period of time?