As is said, if the dose is not known there is no proper way to tell how much to take. But just to be safe, assume that they are on the strong side because they might be. Potent blotter, though with reasonable chance of finding such dose, can very well be 200 ug. I don't think it is a good idea for a first timer to risk going over such a dose by dropping more after an hour.
People without experience can have a harder time telling if they get effects because they don't have a frame of reference yet. Also, even though it is common to feel at least an alert (or more) before the first hour is over, that is not proof that it cannot take longer for other people in some cases.
There is always a next time, I say you are better off risking a dud the first time than going overboard. An underwhelming experience can give you an idea of the quality or nature of the effects and what more could come at higher dose. Just test the water with one and try a higher dose a next time if it does not meet expectations.
How high to go a second time could be estimated based on the effects of 1 hit. Redosing when acute tolerance sets in can complicate determining the potency of a single dose... so if you try to go for a fulfilling experience the first time by dropping more, it can be harder to actually get a feeling for the potency.
I keep thinking what the rush is with some people.

Impatience is not a very good reason to choose a certain dose, is it?
Can you not ask your source what to expect? Remember to ask in the future.
