I've never found any benefit to doing the main quest, just penalties. I no longer even attempt to complete the quest. As far as the time limit, I believe that is based on the people you have to talk to. It tells you to talk to 10 people. Eventually they die off (old age or battle). Once there are no longer enough people left to talk to it fails. This is one of the few logical parts of the quest.
As far as wanting to become your own ruler, the quest lets you become a king at clan level 3. If you don't do the quest you can still start your own kingdom when you reach clan level 4. In my opinion starting a kingdom at clan level 3 isn't a bonus to doing the quest. If Taleworlds thinks you have the capability of being your own ruler at level 3, then they are penalizing you for not doing the quest, they aren't rewarding you for doing it. It would be different if Istania or the other actually gave you a bonus that helped you rule your kingdom, but instead they just give you more problems. As an example, having the Banner and being part of an empire or starting your own empire faction should give you a bonus to relations with all empire culture lords. It could make it cheaper to get them to join your kingdom. It could maybe make it so the other empire kingdoms ally themselves with you or something. Instead the quest just gives you a ****load of enemies attacking you all at once.
As far as the time limit, I believe that is based on the people you have to talk to. It tells you to talk to 10 people. Eventually they die off (old age or battle). Once there are no longer enough people left to talk to it fails. This is one of the few logical parts of the quest.
There are 20+ nobles you can talk to, three or four per faction, so the odds of enough dying in ten years is spectacularly low. A few, like Lucon, will usually bite it really early (he's old as dust) but some are still in their early thirties.
Even if you fail that part of the quest, it just continues to the next part, talking to the two imperial spymasters. and I think it even continues if you don't talk to them, but don't quote me on that.
I generally talk to the lords when I happen to see them, but I don't go looking for them and on the rare occasion where I've completed that first step, the most I'll do is retrieve the banner anyways. I don't talk to the spymasters after getting it because I don't need extra enemies.
It's rather high, if you fight a lot of battles against nobles. Ten game-years was about how long it took me to KiB approximately two-thirds of all nobles of Khuzait ancestry while warring against the Khuzaits.