Apparently, it's different for everyone. In this Quora thread on the topic, the responses range from the horrifying to relatively harmless. Brad Poreus' five days of pain, muscle cramping, incoherence, pouring night sweats, delirium, and narcotics may seem like a nightmare worthy of Hunter Thompson, but it represents the worst case for most people who get bitten by a black widow. The best case? The venom doesn't penetrate a callous and the bitee suffers no ill effects.
Despite their reputation as North America's deadliest spider, black widows are unlikely to kill anyone. In fact, the common treatment for black widow bites simply treats the symptoms--mostly pain and cramping. While there is an antivenin, most hospitals don't have it on hand and it has side effects that can actually be worse than the bite. Unless your bite appears life-threatening, the doctors will probably just treat your incredibly uncomfortable symptoms and send you on your way. You know, to moan and writhe in agony for the next four or five days.
The good news is that not many people are bitten by black widows every year, and practically no one dies of spider bites, much less black widow bites. On average, four people die in the US every year due to spider bites and none of those deaths are caused by black widows. The most recent black-widow related death I could find was in Spain in 2003.
But there's even better news.
Turns out that avoiding black widow bites is actually pretty easy. They're very timid animals, and only bite things as large as people when cornered or threatened (Brad's bite happened when he jammed his foot in a black widow-infested shoe that happened to be filled with the spider's egg sac).
So what should you do if you're afraid you've been bitten by a poisonous spider? It's always a good idea to kill the spider, then bring its remains with you so that the medical professional can confirm that you were actually bitten by a poisonous spider when you go to the emergency room.