Only 6 people have gotten the platinum so far in Ratchet. Their times were
20h
29h (played yesterday for the last time, maybe on a second run?)
17h
20h
23h
<1h (obviously a bug with the tracker)
Looking at the people that got the Gold trophy for finishing the story, but not the Platinum, the playtimes were
15h
11h
25h
26h
22h
19h
12h
11h
It's a huge discrepancy. Don't know if the 11h/12h folk rushed through the game and cutscenes, if the tracker wasn't working, or if there's any other reason, but they look out of place. Looks like it'll take around 15-22h for the average player to complete the story.
Still a small sample size with only 28.5% of people having the Gold, and there are 10 trophies (excluding the platinum) that have a higher rarity than that. There's another player that has not gotten the Gold yet, but has what appears to be the third to last story related trophy, and clocked 26h into the game.
On a personal note, I got 22h out of Miles Morales (still not 100% in NG+) and 26h out of Ratchet PS4, so I'll probably take a while before I'm done with Rift Apart.