I think a better metric would probably be to use the total word count of your script and divide that by the average word-per-minute reading speed of an adult. Therefore a 5,000 word script divided by an average reading speed of 250 words-per-minute results in about a 20 minute playtime.
This is the metric I'm using while working on my own text-based game.
I'm not counting code, and since most scenes have 2-4 options (at least) to vary the plot or scene, I also divide my word count by 4 to give me a rough idea of the amount of words a player is likely to see on any given playthrough.
For instance, my introduction / tutorial has 10,000 words so far, but still has a lot of variation. So I'm assuming most players will see about 2500 words or slightly more, so about 10 minutes is what it will take most people to get through the intro of the game.
It's not fool-proof, but it at least gives me a measurable metric.
I'm sure it may set of some red flags, because 99.99% of people that say this are clueless or scammers. But I genuinely am gonna aim for an insane level of freedom. Countless variations of the same scene with changes based of character stats or appearance or relationship level. Truthfully I had been having a conversation with someone and when I discussed how I was eager to see the future and advancements of games, I was basically laughed at saying that such a high level of freedom was never going to happen.
You've already gotten a lot of good advice on this, but I'll add some more.
I think the draw of ALL text-based games is their ability to deliver a level of freedom impossible in other game mediums. With a text-based game, you can have lots of variation for (mostly) cheap. AAA games all have to hit the exact same beats, and have minimal branching due to the cost of animating, voicing, all that content.
To use my intro/tutorial as an example, a AAA would almost NEVER let the tutorial branch (closest I can think of is the original Dragon Age), but it costs me very little but writing time.
All that said, you have to be careful about EXPONENTIALLY increasing your writing time.
Some things may still have too high a cost, even in a text game, due to the increased workload. Remember that every major variable you have is a MULTIPLIER on your writing time.
So, if the MC can be a man or a woman, that's 2 variables. Two versions of each scene. Fine.
Now, let's say the MC can be gay or straight. That's 2 variables. But combined with the gender variables, that's 2x2 or 4 versions of each scene. Okay.
Now let's say you've let the player choose whether the MC is a nice-person or a jerk as a personality trait, and want to reflect that in each scene. Another 2 variables. 2x2x2 = 8 versions of each scene.
And we have to have the plot affect the scenes! We need to have a different outcome for the (let's say two) choices the player could had the MC make in the last chapter before this scene. 2x2x2x2 = 16(!!!) versions of each scene if you are trying to make them truly unique.
I could go on forever, but that's a good example of how quickly it can balloon out of control.
And it brings up another point - player's will not KNOW how much your game branches without you sign-posting it to them or if they replay. (And sadly, it seems it is a small minority of players that replay text games. Choice of Games collected statistics on it, and the number is depressingly small. Maybe it's better for porn text games.)
So let's say you've released an update and it has 60,000 words, people might get excited, because, wow, that's a novella, almost 75% the length of an actual paperback novel! But . . . that's because you have all those variations - 16 unique versions of every scene. So the amount of content a supporter will experience from your update is about 4000 words . . . or 16 minutes of new content. In this example you damn near wrote a NOVEL for your update, and people will be bitching that it took them longer to download the update than play through it.
Going back a bit, think of the same scenario, but you just had 2 unique versions of each scene. 30,000 words of content for a support who updated, or 2 HOURS of play time.
Just something to think about.
Do not take it the wrong way, I am not laughing at you or trying to be mean, if anything I am offering you a few suggestions so that you can plan better and, most likely, scale down your project to a reasonable level (at least at the beginning), and that is to create a Game Design Document.
Winterfire is right.
The best way to combat scope creep and make sure you can complete your game is to have a detailed Game Design Document.