How long is it okay to make a player wait to rejoin the game after losing a character?

As I peruse RPG forums or read articles, I’m struck by the number of times I’ve come across some variant of this scenario:

Rob Roleplayer finally gets to play in Mr. Legendary GM’s totally awesome campaign. Much fun was had by all and Rob Roleplayer remembers, with fondness, his character imprisoned for several sessions until the other PCs finally got around to rescuing him.

Several sessions? Really? I’d have a mutiny on my hands if I made someone sit out that long! At the very least I’d have to bring a PC in from the Adventurer’s Temp Agency so the affected player had someone to play in the meantime.

(Granted, most of these stories happen in a nostalgic college setting, where there was probably a lot going on around the game to keep the sidelined player happy, but still…)

As a GM, I always try to get new PCs in ASAP, usually within the same session or the next. It’s never occurred to me to make a player show up to a game with the possibility of not getting to play. If I thought the character was going to be out of commission for some time, I would allow the player to bring in another character, with the option of choosing one or the other when the initial character is made available again.

So today I throw the question out to all you GMs reading this: have you ever sidelined a player for more than a session without allowing the player to bring in a substitute for the duration (assuming the player is at the game – sidelining PCs because a player can’t make it is different)? If so, how did your players handle it? Would you do it again? What do you consider a fair period of sidelining?