I believe that there are many factors that would contribute to your question such as weight, age, and any existing health conditions you may have. I have found some very good information on the following link. It may help you find your answer.
You will not find the exact answer you are looking for anywhere as it depends on a lot of factors, mostly to do with the operators individual characteristics and health background. The short answer however is that it is likely to be a very short time if you are really requiring the operator to stand still. I would guess 15-20mins max (but that is just a guess). When standing the muscles are required to do considereable "static work" which means that the blood flow is reduced and the muscles will fatigue very quickly. Rubber mats will not really help in this case. You need to get the operator moving, even short distances, to get the blood flowing. I think you need to go back a step and ask yourself why the operator has to stand still for long periods in the first place.
I don’t know where you are located, but in Europe the regulations require that a "suitable seat MUST be provided to every worker who task is, or can be, done seated". No such regulation in the US but still good advice to follow unless large forces are involved, in which case it should be done standing.