Why shoudl there be a time frame? You move when you are ready. When your BR is too big for th elimit your on, Move up, if your game is better suited for another level, then move to that level. Talking about time frames to move up is like talking about stop wins/losses, they have little application unless you are doing it intentionaly for a good reason.

An "average winning player" will have a WR of about 3 bb/100, and a SD of 20 bb at Limit, the extrapolation math shows that you need about 300 bb's to cover any swings you may have, and this is on the liberal side, meaning you can see swings bigger than that. What this means to you is that you shouldn't be at a limit without 300 bb's in your bank, and you should probably drop down when you hit 200, usually giving you 400 bb's for the next limit down (usually). Moving up is just a function of comfort when you have the 300 bb's. Don't move until you know you can play in that game comfortably.

I seriously don't think your bankroll can be "too big" for a limit. Unless your playing at the micros, there really isn't a rush to move up. Make sure you can beat the game and build a big enough BR for the next level. Also sometimes it may be more profitable to play at the smaller limit if the games are definately softer.