How much time you should be spending in a game based on the cost is very subjective. Personally, I think if you get one hour per €, that is a good enough investment for me. For games, that is low, but compared to other forms of entertainment, that tends to be very high (theme park, movies, concerts, ...).
The amount of time you can spend also depends on the style of game. For online multiplayer games, the amount of time you can spend on them is theoretically infinite. This game is a single player RPG, so by it's very nature the maximum time you can spend is more limited.
I have taken a look through my Steam library, and I happen to have KotOR 2, for which I have done a single playthrough. The game has 43h on record. I'm sure, if you were to look at similar games with a relatively rigid storyline, the amount of time for a playthrough would be similar. Other examples of this would be Neverwinter Nights, Icewind Dale, ... With that in mind, I would say that Solasta scores ok.
Also note that there are games that do much worse. I have playthrough of Dishonored in 27h and one of Bioshock in 16h...
Having said that, I think there is an area where Solasta does fall short, and that is in replay value. There is not enough variation in the classes to make the playthroughs very different, unless you try an oddball idea like a 4 wizard party. The game really needs more classes to spice up replays.
And unfortunately, it looks like classes will be added in the form of DLC, which will make the game expensive...