There' no real payoff in letting it sit too long - in fact it could lead to problems.
I don't know what the "half life" is, but in the real world it seem to do 90% of its thing in the first 15 minutes or so.
Here are two problems that may occur if it's left on too long... As mentioned it works by acting on the calcium and a few other components in concrete, most of these dissolved elements are held in suspension in the acid fluid, but if the acid dries out they're deposited right back onto the concrete, where much of it will re-adhere. It looks like a dusty powder, some can be easily vacuumed or rinsed off, but much won't, it will continue to stick, but not that well. So you epoxy over it, and the epoxy is really sticking to the dust, not the substrate. When the dust eventually seperates from the substrate, the epoxy can fail with it.
Second potential problem... Left to sit some of it will seep and penetrate down into the concrete, and although it seems to weaken pretty quickly, it still has some bite. The surface is eventually rinsed and coated, but encapsulated is a bit of active muriatic acid. The rains come and hydrostatic presuure force some of the acid back to the surface where it's stopped by the backside of the primer coat. There it works on the concrete again, this time dissolving some of the material that the epoxy has adhered to, potentially leading to the coating lifting from the concrete.
So my advice is to let it work for about 30 minutes max, then fully and completely neutralize and power wash.
