Wow those are big questions!!
Will the nap get shorter? Not necessarily. Some LOs will naturally shorten their naps, others will need it shortening for them (ie waking them up) to preserve night time sleep. A small number may keep a long nap right up to the point of nap dropping but those LOs would generally be having much shorter nights as a result.
There is a wide range of ages for the 1-0 to happen, and it can be a long drawn out process for many. I would say average age is probably somewhere around 3 for the nap to go, but can be anywhere from maybe around 2yo or even slightly younger for very LSN LOs, to up to 5yo! It very much depends on sleep needs and lifestyle, for example in some cultures many LOs would stay up very late into the evening so may keep a daytime nap for much longer, whereas in places where LOs are usually in bed for 7pm or thereabouts the nap may go sooner and sleep may be consolidated into a longer night.
You know the nap is too long when you have problems with night sleep - so early wakings (where LO is happy and raring to go for the day), long happy NWs or problems falling asleep at bedtime leading to short nights.
So in short - the only thing you can really expect is that at some point the nap will go away
. How that happens is very variable but please post here any time you run into problems and we will try to help!
