My LO is 15.5 months and we are in the process of transitioning from 2 down to 1 day sleep.
We have been following the SOS routine timings loosely from around 4mo, and this has worked well for us.
Current 2 sleep routine has him awake for 3h45m between naps, and sleeping no later than around 4.15pm to achieve a 7pm bedtime. This works perfectly - although the afternoon nap is often cut short due to the awake time bringing him close to 3:30 by the time he goes down - and he sleeps soundly until 7am most nights (unless he is unwell, teething etc).
He has been sleeping roughly 1h15m at each of these nap times, so roughly a total of 2-2.5hrs sleep over the entire day.
Now that we are moving to 1 sleep, he seems to only be sleeping for the 1h15m. I had assumed he would automatically stretch to a ~2hr nap, but it doesn't seem to be happening. I have tried leaving him in his cot until we get to the 2hr mark, and he doesn't cry or whinge, but on the monitor I can see he fusses around and doesn't end up going back to sleep. Sometimes he lays down as if he is trying but doesn't seem to be able to get there.
Is this just a "wait it out, and he'll get it" situation and the day sleep will naturally stretch out longer once he realises he's not getting another sleep later on, or should I be going in to help him resettle. He has a dummy but it doesn't interrupt his sleep if it drops out. If he wakes he just grabs it and pops it back in if he wants it. He is put down awake and he rolls over and self settles at all other sleep times so I'm hesitant to interfere.