I've got some (actual Proseal brand) that has a ~10 year old date on it, but it's a 'new' container that I've kept in the freezer the entire time. I just used some for a minor project & it took 2-3 days to cure in temps ranging from low 40's to low 60's (during the day when I had the heat on in the shop). The last small batch I mixed (spread on the inside of a fairing intended to be a scuffing protector) was mixed a bit light on the activator, & never cured until I rubbed more activator into the surface. Note that in that case, I was just filling the weave in the fiberglass fairing.
Charlie