That All Depends . . .
Many cars have intermediate shafts that begin making noise when the steering wheel is turned (usually a low speeds when the wheel is turned more).
The intermediate shaft is a short shaft that connects the steering wheel/column with the steering rack that actually turns the wheels. It usually has a universal joint on each end and a sliding sleeve on one end. Sometimes what happens is that the sliding portion loses its lubricant (grease) or it dries out and then the shaft pops or clicks while sliding slightly in and out.
I see on your Rav-4 that the shaft actually is purchased as two pieces, shaft No.2, with universal joints and shaft No.1.
2006-2008 Rav-4s are the vehicles that sometimes have this problem and Toyota says to replace both shafts and a hose clamp. I see that one of the shafts (No.1) has a newer part number. Perhaps the part was revised.
"The dealership says the intermediate steering shaft, which will cost over $500, but that it can wait - I can drive it safely for awhile. First, what the heck is "awhile"? And is he correct - can it be driven safely? How will I know when it is no longer safe?"
I don't want to advise you incorrectly and have you get hurt or killed, but there may not be a big hurry. I believe on GM cars that it's just the sliding part of the shaft that binds (not the joints or attachments) and it's usually just more an annoyance than safety issue, but is that what's making noise in your shaft ? I can only guess that it is. I'd ask a different Toyota mechanic what is actually making the noise and does it present a safety issue ?