DD is 18 months. For a while she's been down to 1 nap and it usually hits around 12:30-1:00... the past couple weeks we had rough nights - multiple wake ups and she was getting to bed later than normal. I made a concerted effort to get her to bed on time and remind her that night-night means going to sleep on her own. The past 3 days she has slept through the night for 10.5-11 hours, which is normal for her.

But nap time is totally out of whack and I can't understand why. She woke up just before 7 this morning and by 10:30/11am she was totally ready for a nap. This has happened the past few days as well - and even worse - she's not napping as long and then she's cranky by 4/5pm 

I've got her eating lunch right now, but she's totally ready to conk out.

What time do your toddlers go down for their nap? Should I just let her nap at 11 or should I try to keep her up till 12:30?

We're up at 6:30 or 7:00am (Mommy prefers 7) and we have lunch around 11:00 or 11:30 and I try and put them down around noon or 12:15.  If i'm lucky they sleep until 1:45 or 2:00.  I try and get them into bed around 7:15 or so at night. (BTW - they are 20 months).
I have a friend with a 15 month old and he's up at 6:30 and down for nap at 1000 until noon.  He usually stays up all afternoon and goes to bed at 6:30.
So I think there are lots of different schedules that can work.  Maybe it's just a phase? I do think that at this age they are supposed to have 13 to 14 hours total - is she getting that much?

DS is almost 19 months.  During the week when he is at daycare he goes down for a nap around noon and usally sleeps until 2-2:30.  On a rare occasion he will sleep until 3.  He goes to bed at 8 PM and usually wakes up between 6 and 7.  So he gets 12-13 hours of sleep a day.

On the weekends when he is home all day sometimes he will get 2 naps.  If he wakes up around 7 he starts acting sleepy by 9 or 10 so we will put him down and he usually sleeps for an hour.  Then we will put him down for another nap around 2 or so or whenever he seems sleepy again and will sleep for an hour to and hour and a half.  So its about the same total naptime as he gets at daycare but is just broken up into 2 naps. I imagine he gets sleepy around 9 or 10 at daycare everyday but they have a schedule they follow.  It seems to work for him and he doesn't get thrown off on the weekends when he gets 2 naps.

So if you don't want to do the two naps I would just force her to stay awake until the noon time nap (unless she seems really exhausted and needs to go down) and she will probably take a longer nap then.

I do lunch at 11:30 then bed at 12 and leave her (17 mos) in there til almost 3. Sometimes she cries during the nap but I don't go in and she settles. She sleeps about 2 hours then plays around. Today she had her pants off!

She might just be going through a growth spurt (or suffering the blame-everything-for-it-teething?), both which can mess up their sleep schedules. I guess I will say you should decide which is more important for your family overall - stick to the schedule that gets her a long night's sleep or add back another nap on days she seems especially tired, even though she might stay up later those nights. I don't think there's a right answer - just a matter of tinkering and seeing what keeps you sane!

To answer the question though -this is my 18mo DD's typical schedule:

Wake 6:30-7am

Nap sometime between 11 and 12:30 (roughly 1-1.5 hours and she naps longer if I put her down earlier, but I have to pick up DS at preschool twice a week and that pushes her naptime toward 12:30 some days)

Bed between 6:30-7

So about 13 hours total sleep. She never STTN, but that's something we've chosen not to deal with yet. (Soon!)

Good luck! Sleep stuff is baffling.