A little campaign help

[Buufreak]

So, my party is currently 7, been going since 3, and I think they are getting a good feel for the world. They have come face to face with a BBEG, fully understood they were outmatched, and saw him as a goal to beat. That made me super happy, to be honest, because my party usually has a tendency to rush in head first, not assess danger, and get killed. So this brings me to my question: How long SHOULD I string them along with this guy before giving the party a fair chance at beating him?

[Extra Anchovies]

As long as you want them to? A lot of games have the main villain revealed early, but they're too powerful for the PCs to take down. You could have an OotS-style game, where the PCs keep getting wrapped up in sidequests but with the distant goal of "gather enough power and resources to defeat [BBEG]".

[Keltest]

Ultimately I think that should be up to the players. They know who he is, and have a taste of what he can do, so it should be up to them to decide when theyre ready to make a go at him. Whether or not theyre actually right can then determine how the meeting goes and whether or not its the final boss fight or if he escapes, or if the party gets slaughtered. Have NPCs give their opinions on whether or not they have the ability to face him and win, or if there is something else they can/should do to better their chances.

As a general rule, don't plan too far ahead. The PCs WILL inevitably go off the rails somewhere and force you to remake your plans from there, so getting too invested in one scenario is pointless.

[Draconium]

Here's a question - have you set a level, of CR, for the BBEG yet? Because I generally find a good BBEG has an EL/CR about two levels higher than the average party level when the final showdown occurs. If you've set one up already, I would tentatively plan on having the party face him at his CR -2. Of course, the party probably will go off the rails eventually, so don't set it in stone - also, the BBEG can always level up as well.

If you don't have his CR yet, then I would recommend waiting until the players believe they are ready to take him on, then determine if they actually are from there. Remember, the fight against the BBEG needs to feel like a challenge, so put it as a difficult fight for their level, but not impossible.

[Geddy2112]

Remember that a solo BBEG or monster is usually an all or nothing fight:either a TPK because it is too powerful, or a cakewalk because the party can be liberal when throwing X/day abilities and will quickly and easily mop the floor with said opponent.

A CR=party is only about 25% of X a day resources for the party, so the encounter should be higher, not just in the CR of the BBEG, but in the overall challenge. Without minions, the BBEG is a save or lose away from death. You can end the campaign and have the fight at any level, but keep in mind the higher level the party is the more likely they are to just nuke the BBEG. Make the BBEG tougher, but also have minions/traps/environment and challenges beforehand to wear the party down a bit. This way, you won't have to have a CR too much higher(wipes the party) but it won't be CR equivalent where the party wins instantly.