How long you can let this go depends on how much you drive it on salted roads, in rain to a lesser extend, and the frequency and size of bumps you hit.
Putting money into it, depends on its purpose. You need it structurally safe and sound, engine and tranny working reasonably. Body rust, meh it's a pickup truck, as long as local ordinances don't prohibit having *rusty* vehicles I wouldn't sweat the body rust until it too becomes structural for example the rocker panels, you don't want them falling through when someone steps on one which is an injury liability issue.
Keep in mind that just because you see that rotten cab mount, that doesn't necessarily mean you don't have a bad ball joint too, or did you check for ball joint play? If it has the original BJ's then I'd imagine it's due for new ones, and that IS more of a safety issue if the knuckle (and wheel) comes off driving. If it needs BJ's, fortunately they're inexpensive to DIY if they are separate from the control arms and in that case may not even require a wheel alignment, while upper control arm replacement usually does throw alignment off.