Typically a 1.5h nap is a good enough nap to last LOs through their next A time. Unless there's something going on, like a cold...or discomfort from shots, and their sleep is fitful, they might just have a rough day.
Towards the end of a sleep cycle, LOs get into REM sleep. They're coming out of the deep sleep and comparatively, REM is a lighter sleep (part of the reason why OT naps are only 30 mins, as this is typically when the REM phase starts).
Here's a really good link that helped me understand sleep...am actually re-reading it now myself
http://babywhispererforums.com/index.php?topic=64515.0If you think your LO might need those few extra minutes at the end of her nap (to make it a full 2h nap), by all means, try and extend!
Hope that helps a bit