average sleep req. for 2.5yr old would be 12.5hrs total, so in theory if she can sleep 12 at night then a 30min nap would be enough total day sleep. But you may find 30mins too little to sustain over a long stretch of days as they can get occumulated OT when starting to skip naps in the same way you can get it when they go from 2-1 nap. So I would try for 1hr nap to start and 11.5hr night.
Does she skip naps herself or is it just that some times she doesn't get one for whatever reason? I wouldn't be tempted to try to cut the nap out, best to let it work itself out. Some days she will nap, others she won't. On no nap days try to stick with quiet time. This can be done in her cot/bed, your bed, even in front of a dvd on sofa. Wherever you can get her to stay fairly still and rest for as long as she will. Your aiming at trying to get a min of 30mins preferably 45mins rest but anything more is more than welcome.
If you find you start to get some OT from the skipped naps then try putting her to bed 30mins earlier in the hope she will extend her night sleep and do 12.5hr night.
