With Broken Steel, technically a character can last indefinitely. However, I always find a point where I loose interest in a character. Here is usually a character life cycle for me:
1. Inspired (concept created)
2. Figure out definite traits and factors (Appearance, gender, morality, opinions)
3. Exit Vault 101
4. Find a "home"
5a. Try to survive
5b. (Or) use cheats/glitches to amass certain things for a role play at expensive of hardship sometimes
6. Do whatever (according to character's own goals)
7. Main Quest
8. (Broken Steel?)
9. Post-Main Quest role playing "needs"
10. Decline of interest
What makes me bored: Lack of fresh ideas to keep the fire burning, my current inability to mod (Dad does not want me to mod until we get a game-able desktop) which hinders roleplaying, reduced to finding random things
What is your own character life cycle, and what makes you get bored?

I just did everything in the game worth doing, the only thing i havet done is get the experimental merv

When all I do is just create new characters and station them in various places in the Wasteland. I don't do any of the quests, I don't explore, etc.

These days, about 8 minutes in. The first playthrough, it got boring at the very first level-scaled encounter. Then it went from boring to insulting the exact moment I figured out "Dad" wasn't going to take no for an answer. Then it got worse when I found out Elder Lyons was essential. I quit by the time Liberty Prime showed up.

-Nukeknockout

To be honest, Fallout 3 got boring after I've done all the quests the game had to offer. I swear, Oblivion had a lot more quests than this one. There are things in Oblivion I haven't done yet since the game's relesae for Talo's sake!

Fallout 3? Yeah, I pretty much did everything.

After a while but then I come back to it like I am now waiting for the Creation Kit for Skyrim to be released.

What I did was just simply start a new game. But this time be mean and rude. Kill everyone and pick the worst lines in conversations. Blow up megaton but kill everyone in it first. I'm telling you its so much fun

I got bored about level 28 and over a hundred hours in. Not too shabby!

I've recently got bored with the metro tunnels and DC. Haven't explored DC a lot, mainly just for quests. But fought through one of them and started thinking "more feral ghouls" then got to Vernon Square I think and just thought "more ruins to fight super mutants in, more abandoned buildings with loot" and just turned around and went back through the tunnel. Went off to do the Broken Steel quests.

Round about the time I accept Wasteland Survival Guide or Those!
But I play on though.

Currently on my 4th or 5th playthru, with about 80 hours on each. I can't get enough of this game...and I actually just got The Pitt DLC. Wasn't the best but it added another couple hours of play. I'm trying to get 100% of the achievements on xbox. I usually do one playthru, do a playthru of Vegas and go back to FO3. I'll never get tired of this game.

I don't get bored with Fallout 3 really, I deleted the Broken Steel quest because I thought it svcked and ruined the games ending and created some plot holes with the story, it all just feels like it was tacked on