[impendia]

I think I read somewhere that the virus has a half-life of roughly 6-7 hours on cardboard. I've also read "24 hours" as a guideline for how long the virus can last. Apparently it depends on conditions such as temperature, sunlight, and humidity. (In particular, direct sunlight and warm weather apparently cause the virus to die faster.)

Although I'm not an epidemiologist, I suspect that your probability of catching the disease from a package decays roughly exponentially with time.

Personally, and as someone not immunocompromised, I've been setting my packages aside (in my apartment) and washing my hands thoroughly afterwards. In cases where I'm particularly eager to get the contents, I'll open them after a day and not worry too much. Otherwise, I'll let them sit for two or three days.