I tend to just guess based on the amount of content. Here's the truth: even if you guess the playtime correctly, it will still not be accurate to most people playing, due to a number of factors, including:
* Number of reloads due to party death.
* Strength of the party and whether a player is using the recommended party or not (it's easy to cheat in FRUA).
* What level the player has chosen to play (easiest will likely be faster than hardest).
* The amount of time the player spends exploring, and whether they bother or not.
* How long someone takes to make decisions in combat.
* How many combats they fight (choice of direction can alter this dramatically).
* Whether or not the player uses "Quick" in combat or fights them manually.
* How long a player spends going back and forth between exploring and their safe base (like a town).
* How long a player spends equipping items, having them ID'd, selling stuff, etc.
* How long it takes a player to find the place they need to go. I've been known to miss something for an hour that was two steps away when I started looking for it.
Point is, there are a lot of things out of the control of even the designer. All you can do is guess. You'll still be wrong, probably.