The leveling and "raid ready" timelines have been completely inverted from the BC and Wrath models. In the BC and Wrath expansions, it took longer to level to the new end cap level (70 and 80, respectively) than it took to gear up to start raiding the new tier. However, players would be well advised to change their expectations on how long it will take to be ready to join a raid group after they hit level 85 in this Cata expansion - it takes much longer to gear up to be raid ready than it takes to level from 80 to 85.
Depending on how well players aligned themselves while leveling to the faction reputation gains they need in order to purchase the blues or epics, they may have a short or long road of faction reputation grinding ahead of them before they have those requisite pieces to hold their own in heroics (the epics translating also into pre-raid readiness).
If they managed their gold well, they will be in a great position to purchase mats or finished crafted pieces; however, there is a chance that they may have some significant grinding to do in order to finance crafted gear and certainly in order to maintain a good supply of flasks which are easily selling for 250g a piece.
Of course, with so few guilds currently unlocking the new fish feast, every raider needs to either cook their own 90 stat buff food, or buy stacks of it. They will need these in the heroics anyways.
Last but not least, there is surviving the heroics over and over in order to get those requisite i346 pieces in order to perform at an entry-level in raids. A tremendous benefit of how Blizzard has set up the end game pre-raid is that rather than spend a lot of time practicing leveling and little time practicing raiding like in BC and Wrath, in Cata we spend very little time practicing our leveling skills and spend a lot of time practicing our raiding skills in fabulously interesting boss fights in the heroics. In effect, we are spending our time practicing the skills we need to be successful for the majority of the expansion (unlike in Wrath and BC).
While it will take most people only a small handful of days to reach level 85 from level 80, it will take them a much longer time to run the regular dungeons to prepare for heroic dungeons to then run the heroic dungeons to prepare for raids.
Raid leaders and guild officers are well-advised to manage these expectations to reduce the chances of people getting frustrated with an unexpected delay in gratification.
As a rule of thumb, I tell people that if the random dungeon finder tells you that you are not geared enough for heroics, you aren't and you should do more regular randoms. Also, if you do not have at least an i346+ average item level on your character sheet (and that doesn't mean stacking items in your bags that are not your main spec!), then you are not ready to start raids and be helpful to the raid group. Btw, items in your bags are averaged into your character sheet Item Level average - putting them in your bank and checking your average item level when the only gear on your character is worn gear will give you a more accurate average.
There are no hard and fast rules and we all hear about the ultra guilds that cleared Naxx wearing T6 still. Sure. However, for the most part if someone is not wearing the epics from reputation and crafting, I don't believe they have put in enough time to get ready to join the rest of the raiders that have. They are also the ones that have not read up on the fights.
Time in preparation is never wasted.