I find those people who extend A times gradually by 10-15mins each fortnight tend not to be the people who post about things going awry. People who wait for the signs often miss them - they are:
- baby sleeps shorter naps (they often gradually get shorter from 1:30 to 1:25 to 1:20, etc.)
- baby wakes frequently or for a long time, generally happy, in the early hours of the morning (3, 4, 5am)
- baby takes longer to fall asleep for naps
Babies can get really in the habit of sleeping at 1:30 A time if they do it for 1.5-2months then they seem OT if you increase their A time by even 5-10mins and really they are capable of staying up and will sleep better if they do but people get very scared of this type of short-term OT without knowing about the chronic OT they're setting up with lots of shorter UT naps.
Long story short, just increase gradually every couple of weeks and if he sleeps a 30min OT nap, be there to resettle and know he'll grow into that A time pretty soon or go back to what was working again for another few days.