It's similar to Steins;Gate, it's basically the true route, with endings branching out from every arc in the story. So whether you go for true or not, you'll be going through most of the story, just skipping the character-focused plots that pop out in the different endings.
The game doesn't keep track of time, so I have no clue how long it took. And while it's slow in the uptake, once it gets going it's amazing and has lots of action and twists, so I don't think you'll get bored even if it takes a bit longer than 20 hours (that's a good guess though).
Knowing you I think it's okay to skip the endings in favor of the true ending, but I'll still recommend you read the Tsubaki ending as soon as it pops up - it has an amazing moment. If you allow me to push for one ending, that one.