For me this +10%

duration is very powerful.
It may sound weird but I find it really useful. By the time you get GA monopoly you can have your first golden age. After that you can focus on your happiness and GA bonusses from policies, beliefs and wonders.
In my last game I had golden age all game long (with only few breaks). That could be because of enormous happiness and GA policies. I admit that it was before recent happiness changes but I'm OK with current bonus.
Although, it may be useless if you're not going for synergies it with other things.
I'll try putting it in some perspective.
Since it is a 10% bonus I'm going to compare it to the +10% yield monopolies.
First of all, there is a policy where part of it's ability is +25% GA duration, that's 250% of the monopoly effect, there is also a wonder and a UA that each provides +50% GA duration, that's 500% of the monopoly effect.
Other than this there is a decision in E&D that is pretty much free and provides +25% GA duration. Sure, E&D is kinda out of whack and not recommended, but that decision have never really been considered overpowered.
Comparing to the other +10% yield monopolies (-growth, because I think that one is garbage as well) the closest wonder comparison is the Sistine chapel providing +15% culture in all cities, sure that's 150% of the +10% culture monopoly, quite a lot, but nowhere near close to the 500% of Chichen itza.
For policies there are a few to choose from. There is a +10% science boost in rationalism and there is a +15% gold in piety. Those however are both scalers, which are generally more powerful than policies, and that's all they provide, compared to the tradition policy which also grants a pretty powerful building in the capital.
It is also worth mentioning that they are tied to requirements, pretty easy requirements but still requirements, positive happiness for the science and a temple as well as your majority religion for the gold. Both the gold and the science bonus are also from later era policy trees.
For the UA there really isn't anything to compare to I guess, but let's go back to the original point with some context:
If there was a UA or a wonder that provided you with a +50% extra global production or culture, you would be raging your face off (with good reason), simply because that isn't balanced.
I'd also like to point out that In my last game I had a 100 turn long golden age as well, and that was with my only actual GA bonus being the E&D decision.