Sadly, it's not actually subjective. Math incoming.
The problem with this design is that Mages get a payoff, at the end of the spell casting which is generally proportional to the time invested. Bards either do not, or get far more, and which of the two happens is defined by the circumstances, which makes it impossible to balance.
The Bard finishes their long-cast buff, and it takes effect. Let's assume that the Bard normally gives 100 power. It is now either:
1: "standard strength" (100 power) and applies for only 60% of the fight, allowing people to get "100 * 60%" (= 60 power) effect
OR
2: "extra strength" (let's say 167 power) to compensate for that, allowing people to get "167 * 60%" (= 100.2 power) effect
In the first case, the party loses out when they get ambushed because they only got 60% as much effect.
In the second case, it's even worse:
In a normal situation, a Bard can sing that song in their rotation, and keep it up for the duration of whatever content you experience. The game has to be balanced around the Bard giving that 167 power all the time, because that's what they'll sustain over the longer term, after they get the first one off and enter their standard rotation. EITHER they now have to be nerfed becuase 167 is too much, or you didn't get what you needed in the first place, because if 167 was fair, 60% of it (back to the ambush scenario) wasn't enough (which is where we started).
Time isn't fungible in these sorts of situations. "How long a Bard takes to get a buff off when their party is ambushed" is a system parameter that has to be set to match your intended design. Because Bards, unlike Mages, are a sustain class, not a burst one, you can't offset this by changing the "power" variable to compensate. "The amount of time a Bard takes to get their buff off" is not a free variable. It's a locked-in product of "How much disadvantage should the Bard (and their party) be at in an ambush scenario?" That design decision determines what your system can be, and itself is dependent on what your other larger systems need to function, such as "how powerful can ambushing be, as a general tactic, when considering Caravans, Siege environments, general Guild v Guild, etc?"
Game design is a very deep stack of "this needs to be set based on that, which needs to be set based on the other thing, which is determined by the core design intent of the game", and designers need to be experts and starting at the right point (in defining their core design), and getting all these waterfall of calculations for the various different systems correct based on it.
I can't tell you absolutely what the exact numbers for "ambush disadvantage when the party has a Bard instead of another [Fighter, Healer, Tank, Empty Slot, etc]", or even general "ambush disadvantage" are supposed to be. All I can say is that "song casting time" is not a free variable, and there's very little subjective about it within a larger design.
Something we
can
discuss, and which, upon reflection, is perhaps the actual subjective part, is this:
You can fix the problem I described by making that 167 have a cooldown on it, and setting the duration so that it can in fact only be active 60% of the time, even under normal circumstances. So here's the subjective part: Do we want that?
I don't like the idea of not being able to sustain my songs. Of having my party be fully buffed only half the time, or of having to use less-appropriate buffs for the other half of the time. I feel like this results in a swingyness of the requirements for my party members, as their capabilities vary during the fight, because the power I am able to give them changes. It puts the skill requirement for 'getting songs right' on them, and takes it away, to some degree, from me.
I want to be in charge of figuring out the right capabilities to give my party (by way of buff choices), and keeping up the timing and flow of that while maintaining my other combat actions (I don't intend to play Minstrel (Bard/Bard)), and then keeping track of when they need different capabilities based on the shift in the flow of a fight. This is what Bard is to me.
Someone once suggested "Bard should have very short buffs with very strong effects, so they can watch for the right time to spike their party's damage/defense/whatever". I don't begrudge that person their desire, but I also don't believe that the class that results from it is still "Bard", and I don't want to play the class that does.
When I give my team a buff to suit a phase of a fight, I want them to be able to rely on it, not have them lose the capabilities it granted while they're still in that phase of the fight, because I'm not allowed to sustain it.
But hey. Maybe there's space in the skill tree for that, and I'll just never unlock it. As long as I can spec for 100% power 100% of the time, I'll be fine. Letting you build for 167% power for 60% of the time is another design consideration, but I don't know what other systems that would affect, so I can't say whether or not it would be valid for Ashes. It's a more MOBA style gameplay type, though, and while I don't doubt that's what some people want, I'm not sure it fits the theme and style of Ashes.
But that's my feeling. What's yours? Given the balance requirement, is that still something you'd want?