I think you're trying to establish a pattern. We know that sometimes this can happen quite quickly, where there is this sort of grooming behaviour and this pattern of behaviour starts soon.
I don't know exactly how long of a time frame there should be, although I do think there should be a period of time where you can start to look at the behaviour over time. In the beginning, it can look kind of subtle and then it can become normalized in the relationship and develop into something more significant.
I don't specifically have a timeline to suggest, but I do know that some sort of a timeline in order to establish that pattern would be helpful.